I can reproduce the SEGFAULT on my system.
I'm using Version 0.20_p12172 on Gentoo.

Szenario:
1.) Playing MP3s and M4A files in MythMusic (occurs only when both types are played)
2.) Leaving MythMusic and start MythMusic again.
3.) When it starts plaining now it crashes.

(gdb) cont
Continuing.
2007-03-18 22:05:34.403 New DB connection, total: 2
2007-03-18 22:05:34.404 Connected to database 'mythconverg' at host: localhost
[New Thread -1398207600 (LWP 6369)]
[New Thread -1406600304 (LWP 6370)]
2007-03-18 22:05:34.448 XMLParse::LoadTheme using /usr/share/mythtv/themes/Titivillus/music-ui.xml
[Thread -1398207600 (LWP 6369) exited]
[Thread -1406600304 (LWP 6370) exited]
2007-03-18 22:05:35.718 Opening ALSA audio device 'default'.
[New Thread -1406600304 (LWP 6371)]
[New Thread -1398207600 (LWP 6372)]
[Thread -1398207600 (LWP 6372) exited]
[Thread -1406600304 (LWP 6371) exited]
2007-03-18 22:05:41.095 Opening ALSA audio device 'default'.
[New Thread -1406600304 (LWP 6373)]
[New Thread -1398207600 (LWP 6374)]
[Thread -1398207600 (LWP 6374) exited]
[Thread -1406600304 (LWP 6373) exited]
2007-03-18 22:05:42.755 Opening ALSA audio device 'default'.
[New Thread -1406600304 (LWP 6375)]
[New Thread -1398207600 (LWP 6376)]
[Thread -1398207600 (LWP 6376) exited]
[Thread -1406600304 (LWP 6375) exited]
2007-03-18 22:05:44.439 Opening ALSA audio device 'default'.
[New Thread -1406600304 (LWP 6377)]
[New Thread -1398207600 (LWP 6378)]
[Thread -1398207600 (LWP 6378) exited]
[Thread -1406600304 (LWP 6377) exited]
[New Thread -1406600304 (LWP 6379)]
[New Thread -1398207600 (LWP 6380)]
2007-03-18 22:05:50.816 XMLParse::LoadTheme using /usr/share/mythtv/themes/Titivillus/music-ui.xml
[Thread -1406600304 (LWP 6379) exited]
[Thread -1398207600 (LWP 6380) exited]
2007-03-18 22:05:51.957 Opening ALSA audio device 'default'.
[New Thread -1398207600 (LWP 6381)]
[New Thread -1406600304 (LWP 6382)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1406600304 (LWP 6382)]
0xb68eaf64 in QGList::append () from /usr/qt/3/lib/libqt-mt.so.3
(gdb) backtrace
#0  0xb68eaf64 in QGList::append () from /usr/qt/3/lib/libqt-mt.so.3
#1  0xb65cca57 in QApplication::postEvent () from /usr/qt/3/lib/libqt-mt.so.3
#2  0xb6ef16c4 in MythObservable::dispatch () from /usr/lib/libmyth-0.20.so.0
#3  0xb1a09868 in aacDecoder::run () from /usr/lib/mythtv/plugins/libmythmusic.so
#4  0xb65c39fb in QThreadInstance::start () from /usr/qt/3/lib/libqt-mt.so.3
#5  0xb61c4684 in start_thread () from /lib/tls/libpthread.so.0
#6  0xb605df9e in clone () from /lib/tls/libc.so.6
(gdb)