Full backtrace from above, and important note at bottom:

(gdb) thread apply all bt

Thread 8 (Thread 0xb0682b90 (LWP 18625)):
#0  0xb7fdf410 in __kernel_vsyscall ()
#1  0xb6c4dc07 in poll () from /lib/tls/i686/cmov/libc.so.6
#2  0x081e2546 in MidiObjectALSASeq::run (this=0x86fe548) at src/midiobjectalsaseq.cpp:255
#3  0xb72b3057 in ?? () from /usr/lib/libQtCore.so.4
#4  0xb70684fb in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#5  0xb6c57e5e in clone () from /lib/tls/i686/cmov/libc.so.6

Thread 7 (Thread 0xb0e83b90 (LWP 18624)):
#0  0xb7fdf410 in __kernel_vsyscall ()
#1  0xb706caa5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/tls/i686/cmov/libpthread.so.0
#2  0xb72b3924 in QWaitCondition::wait () from /usr/lib/libQtCore.so.4
#3  0x0818f2b3 in BpmDetector::run (this=0x84fae80) at src/bpmdetector.cpp:127
#4  0xb72b3057 in ?? () from /usr/lib/libQtCore.so.4
#5  0xb70684fb in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#6  0xb6c57e5e in clone () from /lib/tls/i686/cmov/libc.so.6

Thread 6 (Thread 0xb1684b90 (LWP 18623)):
#0  0xb7fdf410 in __kernel_vsyscall ()
#1  0xb706caa5 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/tls/i686/cmov/libpthread.so.0
#2  0xb72b3924 in QWaitCondition::wait () from /usr/lib/libQtCore.so.4
#3  0x0818d185 in WaveSummary::run (this=0xfffffe00) at src/wavesummary.cpp:81
#4  0xb72b3057 in ?? () from /usr/lib/libQtCore.so.4
#5  0xb70684fb in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#6  0xb6c57e5e in clone () from /lib/tls/i686/cmov/libc.so.6

Thread 5 (Thread 0xb1e85b90 (LWP 18621)):
#0  0xb734bde0 in ?? () from /usr/lib/libQtCore.so.4
#1  0xb72b3057 in ?? () from /usr/lib/libQtCore.so.4
#2  0xb70684fb in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#3  0xb6c57e5e in clone () from /lib/tls/i686/cmov/libc.so.6

Thread 1 (Thread 0xb57da760 (LWP 18605)):
#0  0xb7fdf410 in __kernel_vsyscall ()
---Type <return> to continue, or q <return> to quit---
#1  0xb70708eb in waitpid () from /lib/tls/i686/cmov/libpthread.so.0
#2  0x08230271 in cm_close (timeout=10) at lib/cmetrics/lindriver.cpp:266
#3  0x0810986c in ~MixxxApp (this=0x8418188) at src/mixxx.cpp:483
#4  0x080f2eac in main (argc=1, argv=0xbfd6d3a4) at src/main.cpp:220
======

Mixxx's output:
Debug: Destroying MixxxApp 
Debug: Write track xml,  0 
Debug: close soundmanager 330 
Debug: SoundManager::closeDevices() 
[Thread 0xad90ab90 (LWP 18686) exited]
Debug: soundmanager->close() done 
Debug: delete soundmanager,  379 
Debug: SoundManager::clearDeviceList() 
Debug: SoundManager::closeDevices() 
Debug: delete master,  380 
Debug: in ~EngineMaster() 
Debug: delete channel1,  381 
Debug: delete channel2,  381 
Debug: delete buffer1,  382 
Debug: delete buffer2,  395 
Debug: delete view,  397 
[Thread 0xb42d2b90 (LWP 18666) exited]
[Thread 0xb4ff2b90 (LWP 18665) exited]
[Thread 0xb3375b90 (LWP 18667) exited]
[Thread 0xae11ab90 (LWP 18685) exited]
[Thread 0xafe14b90 (LWP 18684) exited]
Debug: delete tracks,  465 
Debug: save config,  474 
Debug: delete config,  476 

(hangs here... see note below)

======

Important Note:
If I exit and just don't touch it, it'll eventually timeout and say:
   Program terminated with signal SIGKILL, Killed.