hi i experience the same problem with the beta3 version. i can also reproduce the crash

my system is a laptop with 2GB ram

$ uname -a
> Linux localhost 2.6.24.4.tex3 #1 SMP Sat Mar 29 16:07:20 CDT 2008 i686 Intel(R) Pentium(R) M processor 2.13GHz GNU/Linux
$ lsb_release -a
> LSB Version:      
sb-3.1-ia32:lsb-3.1-noarch:core-3.0-ia32:core-3.0-noarch:core-3.1-ia32:core-3.1-noarch:cxx-3.0-ia32:cxx-3.0-noarch:graphics-3.0-ia32:graphics-3.0-noarch:lsb-3.0-ia32:lsb-3.0-noarch
Distributor ID: MandrivaLinux
Description:    PCLinuxOS
Release:        2007
Codename:     PCLinuxOS

i dont use a realtime kernel but I run jackd via QJackCtl with the following settings
jackd runs with nice -19

JACK_RELEASE = 0-111-5
JACK_VERSION = 0.111.5
JACK_PROTOCOL_VERSION = 23
JACK_SO_VERSION = 0:28:0

realtime    ON
soft mode ON
priority      75
frames      256
buffer        4
lactency    21.2ms
hardware   alesis io2 

some other libs

qt               version 4.4.0
libvorbisfile  version 1.2.0-1
libvorbis      version 1.2.0
libsndfile     version 1.0.18-0
libalsa2       version 1.0.16-1

mixxx, jack, qt and other libs are manually compiled

gcc (GCC) 4.1.1 20060724 (prerelease) (4.1.1-4pclos2007)

i collected some informations with gdb and the normal logging.
this is the debug logging and backtrace from gdb

[New Thread -1333044320 (LWP 5046)]
[Thread -1333044320 (LWP 5046) exited]
Debug: Midi OK (Workaround not required)
Debug: setupMappings( "/usr/local//share/mixxx/midi/Evolution_Xsession.midi.xml" )
Debug: slotApply crossfader: 1 "SlowFade"
Debug: Load to player2: "/home/olodumare/Documents/audio/remix/mochipet_contest_may_2008/35 - 099_lazyday_kflay_voxx.wav"
Debug: file length  18816000 i
Debug: WaveSummary generation successful for "35 - 099_lazyday_kflay_voxx.wav"
Debug: BPM detection failed, setting to 0.
Debug: Load to player2: "/home/olodumare/Documents/audio/remix/mochipet_contest_may_2008/37 - 108_do_what_you_feel_voxx.wav"
Debug: file length  16856000 i
Debug: WaveSummary generation successful for "37 - 108_do_what_you_feel_voxx.wav"
Debug: BPM detection successful for "37 - 108_do_what_you_feel_voxx.wav"
Debug: BPM 75.8267
Debug: Load to player1: "/home/olodumare/Documents/audio/traxx/Death_to_the_Fascist_Pigs_6.ogg"
Debug: file length  20869726 i
Debug: WaveSummary generation successful for "Death_to_the_Fascist_Pigs_6.ogg"

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1324651616 (LWP 5038)]
SoundSourceOggVorbis::read (this=0x882e8c0, size=8192, destination=0xb10b22f4)
    at src/soundsourceoggvorbis.cpp:129
129                     dest[index] = 0;

(gdb) bt
#0  SoundSourceOggVorbis::read (this=0x882e8c0, size=8192, destination=0xb10b22f4)
    at src/soundsourceoggvorbis.cpp:129
#1  0x0816e6a2 in BpmDetector::run (this=0x85477c0) at src/bpmdetector.cpp:198
#2  0xb6e5a512 in QThreadPrivate::start (arg=0x85477c0) at thread/qthread_unix.cpp:190
#3  0xb6bfe562 in start_thread () from /lib/i686/libpthread.so.0
#4  0xb66994de in clone () from /lib/i686/libc.so.6

(gdb) bt full
#0  SoundSourceOggVorbis::read (this=0x882e8c0, size=8192, destination=0xb10b22f4)
    at src/soundsourceoggvorbis.cpp:129
No locals.
#1  0x0816e6a2 in BpmDetector::run (this=0x85477c0) at src/bpmdetector.cpp:198
        pSoundSource = (class SoundSourceProxy *) 0x882e610
        pos = 20865024
        end = 20869726
        frequency = 44100
        samples = {0 <repeats 8192 times>}
        channels = 2
        bpmd = {_vptr.BpmDetect = 0x821a040, xcorr = 0x88567d0, envelopeAccu = 1.40129846e-45,
  RMSVolumeAccu = 9.72530365, decimateCount = 24, decimateSum = 0, decimateBy = 88, windowLen = 400,
  channels = 2, sampleRate = 44100, maxBpm = 170, minBpm = 75, windowStart = 176, buffer = 0x882ec18}
        data16 = {0 <repeats 8192 times>}
        length = Variable "length" is not available.

i have no log file as gdb didnt write logging into a seprate file even if I adjusted it this way and there is also no core file
hope this helps. peace olo