Starting over, based on this thread :
...And my problem which is the same!
I have the same audio chipset and experienced the same issues...
I hope there is nothing about that chip. I tried with XFCE4 on Slackware 13.1 and now KDE4 on Debian, I found a workaround on XFCE by installing gstreamer and using it instead of xine as the phonon backend.
That workaround apparently doesn't work on KDE.
Something interesting (also in the original thread) :
Software mixing through alsa DOES work since I can play several files in aplay (command line) or xine (gui standalone) simultaneously.
I don't have another soundcard so I need to fix this since the problem explained in the thread is very annoying.
Should I install pulseaudio anyway ?
Switching to OSS ?