I also seem to share the same problem; here is a pair of backtraces of a gdb attach to kmix; it seems to be stuck in ViewBase::setMixSet


#0  __memcpy_ssse3_back () at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:1517
#1  0x00007f960927082c in QString::realloc (this=0x7ffff0265b30, alloc=524272) at /usr/include/bits/string3.h:52
#2  0x00007f9609270d4f in QString::append (this=0x7ffff0265b30, str=...) at tools/qstring.cpp:1534
#3  0x00007f960925eb9d in operator+= (this=0x7ffff0265c90, b=...) at ../../include/QtCore/../../src/corelib/tools/qstring.h:281
#4  operator QString (this=0x7ffff0265c90, b=...) at ../../include/QtCore/../../src/corelib/tools/qstringbuilder.h:130
#5  QRegExpEngine::Box::cat (this=0x7ffff0265c90, b=...) at tools/qregexp.cpp:2535
#6  0x00007f9609260812 in QRegExpEngine::parseTerm (this=0x67bec70, box=0x7ffff0265c90) at tools/qregexp.cpp:3624
#7  0x00007f96092609e1 in QRegExpEngine::parseExpression (this=0x67bec70, box=0x7ffff0265c90) at tools/qregexp.cpp:3630
#8  0x00007f960925edad in QRegExpEngine::parse (this=0x67bec70, pattern=<value optimised out>, len=<value optimised out>)
    at tools/qregexp.cpp:3376
#9  0x00007f9609260cda in QRegExpEngine::QRegExpEngine (this=<value optimised out>, key=...) at tools/qregexp.cpp:1348
#10 0x00007f9609262869 in prepareEngine_helper (priv=0x68d6fc0) at tools/qregexp.cpp:3711
#11 0x00007f9609262a25 in prepareEngine (priv=0x68d6fc0, str=...) at tools/qregexp.cpp:3721
#12 prepareEngineForMatch (priv=0x68d6fc0, str=...) at tools/qregexp.cpp:3726
#13 0x00007f9609262b4b in QRegExp::indexIn (this=0x7ffff0265f90, str=..., offset=0, caretMode=QRegExp::CaretAtZero)
    at tools/qregexp.cpp:4110
#14 0x00007f960c817e37 in contains (this=0x20d0760) at /usr/include/qt4/QtCore/qstring.h:218
#15 ViewSliders::_setMixSet (this=0x20d0760) at ../../kmix/gui/viewsliders.cpp:184
#16 0x00007f960c8160d5 in ViewBase::setMixSet (this=0x20d0760) at ../../kmix/gui/viewbase.cpp:272
#17 0x00007f960c816317 in ViewBase::controlsReconfigured (this=0x20d0760, mixer_ID=...) at ../../kmix/gui/viewbase.cpp:203
#18 0x00007f960c814936 in ViewBase::qt_metacall (this=0x20d0760, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, 
    _a=0x7ffff0266280) at ./viewbase.moc:89
#19 0x00007f960c8177b8 in ViewSliders::qt_metacall (this=0x20d0760, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, 
    _a=<value optimised out>) at ./viewsliders.moc:67

#20 0x00007f96093255e8 in QMetaObject::activate (sender=0x15e5900, m=<value optimised out>, local_signal_index=<value optimised out>, 
    argv=0x7ffff0266280) at kernel/qobject.cpp:3287
#21 0x00007f960c834745 in Mixer::controlsReconfigured (this=<value optimised out>, _t1=<value optimised out>) at ./mixer.moc:118
#22 0x00007f960c8347b1 in controlsReconfiguredForwarder (this=0x15e5900, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, 
    _a=0x7ffff0266380) at ../../kmix/core/mixer.cpp:229
#23 Mixer::qt_metacall (this=0x15e5900, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, _a=0x7ffff0266380) at ./mixer.moc:93
#24 0x00007f96093255e8 in QMetaObject::activate (sender=0x15e6630, m=<value optimised out>, local_signal_index=<value optimised out>, 
    argv=0x7ffff0266380) at kernel/qobject.cpp:3287
#25 0x00007f960c8457d5 in Mixer_Backend::controlsReconfigured (this=<value optimised out>, _t1=<value optimised out>)
    at ./mixer_backend.moc:100
#26 0x00007f960c83cf09 in Mixer_PULSE::removeWidget (this=0x15e6630, index=12) at ../../kmix/backends/mixer_pulse.cpp:755
#27 0x00007f960c83d3e2 in subscribe_cb (c=<value optimised out>, t=<value optimised out>, index=12)
    at ../../kmix/backends/mixer_pulse.cpp:534
#28 0x00007f960ac59f04 in pa_command_subscribe_event (pd=<value optimised out>, command=<value optimised out>, tag=<value optimised out>, 
    t=0x68d8140, userdata=<value optimised out>) at pulse/subscribe.c:58
#29 0x00007f9602c829a6 in pa_pdispatch_run (pd=0x13bbb50, packet=<value optimised out>, creds=0x67da7a0, userdata=0x1630840)
    at pulsecore/pdispatch.c:331
#30 0x00007f960ac3cd5d in pstream_packet_callback (p=<value optimised out>, packet=0x6869360, creds=0x67da7a0, userdata=0x1630840)
    at pulse/context.c:346
#31 0x00007f9602c87064 in do_read (p=0x67da650) at pulsecore/pstream.c:816
#32 do_something (p=0x67da650) at pulsecore/pstream.c:184
#33 0x00007f960aa2bc17 in dispatch_func (source=0x15e16e0, callback=<value optimised out>, userdata=<value optimised out>)
    at pulse/glib-mainloop.c:585

#34 0x00007f960276db3d in g_main_dispatch (context=0x12b5880) at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:2440
---Type <return> to continue, or q <return> to quit---
#35 g_main_context_dispatch (context=0x12b5880) at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:3013
#36 0x00007f960276e318 in g_main_context_iterate (context=0x12b5880, block=<value optimised out>, dispatch=1, self=<value optimised out>)
    at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:3091
#37 0x00007f960276e5a9 in g_main_context_iteration (context=0x12b5880, may_block=1) at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:3154
#38 0x00007f960933b42f in QEventDispatcherGlib::processEvents (this=0x1280340, flags=<value optimised out>)
    at kernel/qeventdispatcher_glib.cpp:422
#39 0x00007f9609fedd7e in ?? () from /usr/lib/libQtGui.so.4
#40 0x00007f960930f852 in QEventLoop::processEvents (this=<value optimised out>, flags=...) at kernel/qeventloop.cpp:149
#41 0x00007f960930fa8c in QEventLoop::exec (this=0x7ffff0266810, flags=...) at kernel/qeventloop.cpp:201
#42 0x00007f9609313e9b in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1008
#43 0x00007f960c8097ba in kdemain (argc=3, argv=0x7ffff0266d98) at ../../kmix/apps/main.cpp:73
#44 0x00007f960c45feff in __libc_start_main (main=0x400730 <main(int, char**)>, argc=3, ubp_av=0x7ffff0266d98, init=<value optimised out>, 
    fini=<value optimised out>, rtld_fini=<value optimised out>, stack_end=0x7ffff0266d88) at libc-start.c:226


** and another **

#1  0x00007f960927082c in QString::realloc (this=0x7ffff0265b30, alloc=131056) at /usr/include/bits/string3.h:52
#2  0x00007f9609270d4f in QString::append (this=0x7ffff0265b30, str=...) at tools/qstring.cpp:1534
#3  0x00007f960925eb9d in operator+= (this=0x7ffff0265c90, b=...) at ../../include/QtCore/../../src/corelib/tools/qstring.h:281
#4  operator QString (this=0x7ffff0265c90, b=...) at ../../include/QtCore/../../src/corelib/tools/qstringbuilder.h:130
#5  QRegExpEngine::Box::cat (this=0x7ffff0265c90, b=...) at tools/qregexp.cpp:2535
#6  0x00007f9609260812 in QRegExpEngine::parseTerm (this=0x16293c0, box=0x7ffff0265c90) at tools/qregexp.cpp:3624
#7  0x00007f96092609e1 in QRegExpEngine::parseExpression (this=0x16293c0, box=0x7ffff0265c90) at tools/qregexp.cpp:3630
#8  0x00007f960925edad in QRegExpEngine::parse (this=0x16293c0, pattern=<value optimised out>, len=<value optimised out>)
    at tools/qregexp.cpp:3376
#9  0x00007f9609260cda in QRegExpEngine::QRegExpEngine (this=<value optimised out>, key=...) at tools/qregexp.cpp:1348
#10 0x00007f9609262869 in prepareEngine_helper (priv=0x68d6fc0) at tools/qregexp.cpp:3711
#11 0x00007f9609262a25 in prepareEngine (priv=0x68d6fc0, str=...) at tools/qregexp.cpp:3721
#12 prepareEngineForMatch (priv=0x68d6fc0, str=...) at tools/qregexp.cpp:3726
#13 0x00007f9609262b4b in QRegExp::indexIn (this=0x7ffff0265f90, str=..., offset=0, caretMode=QRegExp::CaretAtZero)
    at tools/qregexp.cpp:4110
#14 0x00007f960c817e37 in contains (this=0x20d0760) at /usr/include/qt4/QtCore/qstring.h:218
#15 ViewSliders::_setMixSet (this=0x20d0760) at ../../kmix/gui/viewsliders.cpp:184
#16 0x00007f960c8160d5 in ViewBase::setMixSet (this=0x20d0760) at ../../kmix/gui/viewbase.cpp:272
#17 0x00007f960c816317 in ViewBase::controlsReconfigured (this=0x20d0760, mixer_ID=...) at ../../kmix/gui/viewbase.cpp:203
#18 0x00007f960c814936 in ViewBase::qt_metacall (this=0x20d0760, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, 
    _a=0x7ffff0266280) at ./viewbase.moc:89
#19 0x00007f960c8177b8 in ViewSliders::qt_metacall (this=0x20d0760, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, 
    _a=<value optimised out>) at ./viewsliders.moc:67

#20 0x00007f96093255e8 in QMetaObject::activate (sender=0x15e5900, m=<value optimised out>, local_signal_index=<value optimised out>, 
    argv=0x7ffff0266280) at kernel/qobject.cpp:3287
#21 0x00007f960c834745 in Mixer::controlsReconfigured (this=<value optimised out>, _t1=<value optimised out>) at ./mixer.moc:118
#22 0x00007f960c8347b1 in controlsReconfiguredForwarder (this=0x15e5900, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, 
    _a=0x7ffff0266380) at ../../kmix/core/mixer.cpp:229
#23 Mixer::qt_metacall (this=0x15e5900, _c=QMetaObject::InvokeMetaMethod, _id=<value optimised out>, _a=0x7ffff0266380) at ./mixer.moc:93
#24 0x00007f96093255e8 in QMetaObject::activate (sender=0x15e6630, m=<value optimised out>, local_signal_index=<value optimised out>, 
    argv=0x7ffff0266380) at kernel/qobject.cpp:3287
#25 0x00007f960c8457d5 in Mixer_Backend::controlsReconfigured (this=<value optimised out>, _t1=<value optimised out>)
    at ./mixer_backend.moc:100
#26 0x00007f960c83cf09 in Mixer_PULSE::removeWidget (this=0x15e6630, index=12) at ../../kmix/backends/mixer_pulse.cpp:755
#27 0x00007f960c83d3e2 in subscribe_cb (c=<value optimised out>, t=<value optimised out>, index=12)
    at ../../kmix/backends/mixer_pulse.cpp:534
#28 0x00007f960ac59f04 in pa_command_subscribe_event (pd=<value optimised out>, command=<value optimised out>, tag=<value optimised out>, 
    t=0x68d8140, userdata=<value optimised out>) at pulse/subscribe.c:58
#29 0x00007f9602c829a6 in pa_pdispatch_run (pd=0x13bbb50, packet=<value optimised out>, creds=0x67da7a0, userdata=0x1630840)
    at pulsecore/pdispatch.c:331
#30 0x00007f960ac3cd5d in pstream_packet_callback (p=<value optimised out>, packet=0x6869360, creds=0x67da7a0, userdata=0x1630840)
    at pulse/context.c:346
#31 0x00007f9602c87064 in do_read (p=0x67da650) at pulsecore/pstream.c:816
#32 do_something (p=0x67da650) at pulsecore/pstream.c:184
#33 0x00007f960aa2bc17 in dispatch_func (source=0x15e16e0, callback=<value optimised out>, userdata=<value optimised out>)
    at pulse/glib-mainloop.c:585

#34 0x00007f960276db3d in g_main_dispatch (context=0x12b5880) at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:2440
---Type <return> to continue, or q <return> to quit---
#35 g_main_context_dispatch (context=0x12b5880) at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:3013
#36 0x00007f960276e318 in g_main_context_iterate (context=0x12b5880, block=<value optimised out>, dispatch=1, self=<value optimised out>)
    at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:3091
#37 0x00007f960276e5a9 in g_main_context_iteration (context=0x12b5880, may_block=1) at /build/buildd/glib2.0-2.28.2/./glib/gmain.c:3154
#38 0x00007f960933b42f in QEventDispatcherGlib::processEvents (this=0x1280340, flags=<value optimised out>)
    at kernel/qeventdispatcher_glib.cpp:422
#39 0x00007f9609fedd7e in ?? () from /usr/lib/libQtGui.so.4
#40 0x00007f960930f852 in QEventLoop::processEvents (this=<value optimised out>, flags=...) at kernel/qeventloop.cpp:149
#41 0x00007f960930fa8c in QEventLoop::exec (this=0x7ffff0266810, flags=...) at kernel/qeventloop.cpp:201
#42 0x00007f9609313e9b in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1008
#43 0x00007f960c8097ba in kdemain (argc=3, argv=0x7ffff0266d98) at ../../kmix/apps/main.cpp:73
#44 0x00007f960c45feff in __libc_start_main (main=0x400730 <main(int, char**)>, argc=3, ubp_av=0x7ffff0266d98, init=<value optimised out>, 
    fini=<value optimised out>, rtld_fini=<value optimised out>, stack_end=0x7ffff0266d88) at libc-start.c:226
#45 0x0000000000400659 in _start ()