Actually I had the same problem om Slack 9.1 on another computer, but
didn't bother to do any research back then (just for the record..)

When i start KDE from runlevel 3 (startkde) XMMS and other programs
that tries to use OSS (ALSA emulates OSS, right? at least I have a
snd-pcm-oss module loaded by default) can't play music at all, and
programs that can use ALSA have no problems except that only ONE sound
can be played at a time, the other programs just have to queue up and
wait for their turn to play.

Googled at bit around and learned about artsd. Tried to start artsd
manually before starting kde which fixed the latter problem (many sound
sources), but still XMMS would not play (even though i changed output
plugin to ALSA).

When i started KDE from runlevel 4 (GDM login screen) KDE complained
about no permission to /dev/dsp and said that all sound output was
redirected to null. XMMS and other programs using oss (iTunes with
Crossover Office) now worked perfectly, but still only one source could
play sound.

Anyone has an idea about what could be wrong?