Version:           unknown (using 4.1.85 (KDE 4.1.85 (KDE 4.2 Beta2)), Kubuntu packages)
Compiler:          cc
OS:                Linux (x86_64) release 2.6.27-9-generic

Seems like kmix can be confused by appearing sound devices (but doesn't have to, it doesn't always crash).  Unfortunately is the backtrace broken.  Hope this helps anyway.

dmesg says:

[24800.896547] usb 1-2: USB disconnect, address 7
[24800.896553] usb 1-2.4: USB disconnect, address 8
[24839.600521] usb 1-2: new full speed USB device using ohci_hcd and address 9
[24839.781476] usb 1-2: configuration #1 chosen from 1 choice
[24839.784209] hub 1-2:1.0: USB hub found
[24839.786632] hub 1-2:1.0: 4 ports detected
[24840.101631] usb 1-2.4: new full speed USB device using ohci_hcd and address 10
[24840.296710] usb 1-2.4: configuration #1 chosen from 1 choice
[24840.550679] input: Philips Electronics USB Playback and Recording System as /devices/pci0000:00/0000:00:13.0/usb1/1-2/1-2.4/1-2.4:1.3/input/input10
[24840.576736] input,hidraw3: USB HID v1.00 Device [Philips Electronics USB Playback and Recording System] on usb-0000:00:13.0-2.4

According to xsession-errors there's an assertion hit:

QObject: Do not delete object, 'unnamed', during its event handler!
kmix(9215) Mixer::openIfValid: Mixer::open() detected master:  "PCM:0"
ASSERT: "dev->backendObject()==0" in file /build/buildd/kde4libs-4.1.85/solid/solid/devicemanager.cpp, line 164
QObject: Do not delete object, 'unnamed', during its event handler!
KCrash: Application 'kmix' crashing...


This backtrace appears to be of no use.
This is probably because your packages are built in a way which prevents creation of proper backtraces, or the stack frame was seriously corrupted in the crash.

[Thread debugging using libthread_db enabled]
[New Thread 0x7fd8403986f0 (LWP 9195)]
[New Thread 0x40fd0950 (LWP 9424)]
[New Thread 0x417d1950 (LWP 9423)]
0x00007fd83990d621 in nanosleep () from /lib/libc.so.6
[Current thread is 0 (LWP 9195)]

Thread 3 (Thread 0x417d1950 (LWP 9423)):
#0  0x00007fd839942206 in poll () from /lib/libc.so.6
#1  0x00007fd835bf23a8 in ?? () from /usr/lib/libglib-2.0.so.0
#2  0x00007fd835bf26cb in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#3  0x00007fd83b2a517e in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
#4  0x00007fd83b27b682 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
#5  0x00007fd83b27b80d in QEventLoop::exec () from /usr/lib/libQtCore.so.4
#6  0x00007fd82d8d72cb in QCA::SyncThread::run () from /usr/lib/libqca.so.2
#7  0x00007fd83b194362 in ?? () from /usr/lib/libQtCore.so.4
#8  0x00007fd8384763ea in start_thread () from /lib/libpthread.so.0
#9  0x00007fd83994bc6d in clone () from /lib/libc.so.6
#10 0x0000000000000000 in ?? ()

Thread 2 (Thread 0x40fd0950 (LWP 9424)):
#0  0x00007fd83847a2d9 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/libpthread.so.0
#1  0x00007fd83b195349 in QWaitCondition::wait () from /usr/lib/libQtCore.so.4
#2  0x00007fd83c50729c in ?? () from /usr/lib/libQtNetwork.so.4
#3  0x00007fd83b194362 in ?? () from /usr/lib/libQtCore.so.4
#4  0x00007fd8384763ea in start_thread () from /lib/libpthread.so.0
#5  0x00007fd83994bc6d in clone () from /lib/libc.so.6
#6  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x7fd8403986f0 (LWP 9195)):
#0  0x00007fd83990d621 in nanosleep () from /lib/libc.so.6
#1  0x00007fd83990d447 in sleep () from /lib/libc.so.6
#2  0x00007fd83bebcf4f in KCrash::startDrKonqi (argv=0x7fff483d1118, argc=17) at /build/buildd/kde4libs-4.1.85/kdeui/util/kcrash.cpp:412
#3  0x00007fd83bebd88a in KCrash::defaultCrashHandler (sig=11) at /build/buildd/kde4libs-4.1.85/kdeui/util/kcrash.cpp:337
#4  <signal handler called>
#5  0x00007fd83c51ca43 in ?? () from /usr/lib/libQtNetwork.so.4
#6  0x00007fd83c51f74c in ?? () from /usr/lib/libQtNetwork.so.4
#7  0x00007fd83c51f8c7 in ?? () from /usr/lib/libQtNetwork.so.4
#8  0x00007fd83c510991 in ?? () from /usr/lib/libQtNetwork.so.4
#9  0x00007fd83a54bc3d in QApplicationPrivate::notify_helper () from /usr/lib/libQtGui.so.4
#10 0x00007fd83a5539ba in QApplication::notify () from /usr/lib/libQtGui.so.4
#11 0x00007fd83be53f0b in KApplication::notify (this=0x7fff483d1f30, receiver=0x186bc40, event=0x7fff483d1c40) at /build/buildd/kde4libs-4.1.85/kdeui/kernel/kapplication.cpp:307
#12 0x00007fd83b27cd61 in QCoreApplication::notifyInternal () from /usr/lib/libQtCore.so.4
#13 0x00007fd83b2a53d9 in ?? () from /usr/lib/libQtCore.so.4
#14 0x00007fd835beed3b in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#15 0x00007fd835bf250d in ?? () from /usr/lib/libglib-2.0.so.0
#16 0x00007fd835bf26cb in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#17 0x00007fd83b2a515f in QEventDispatcherGlib::processEvents () from /usr/lib/libQtCore.so.4
#18 0x00007fd83a5dda6f in ?? () from /usr/lib/libQtGui.so.4
#19 0x00007fd83b27b682 in QEventLoop::processEvents () from /usr/lib/libQtCore.so.4
#20 0x00007fd83b27b80d in QEventLoop::exec () from /usr/lib/libQtCore.so.4
#21 0x00007fd83b27dcbd in QCoreApplication::exec () from /usr/lib/libQtCore.so.4
#22 0x0000000000449f8c in main (argc=3, argv=0x7fff483d2ce8) at /build/buildd/kdenetwork-4.1.85/kopete/kopete/main.cpp:102
#0  0x00007fd83990d621 in nanosleep () from /lib/libc.so.6