This is most likely because of 'a big alsa update' between 2.6.12 and 2.6.15. The simple solution is to start playing an audio file, run alsamixer from the command line, and try cranking up and un-muting the various volume channels. I've been bit by this a couple of times, and I was bit by this when I went from 2.6.12 to 2.6.15.
It seems that with my onboard sound chip the number of things that could be set changed, and the settings of those things got somewhat scrambled... Some things wound up muted... This is easily reproducible for me (eg, switch kernels), and isn't Debian specific (It has bitten me under Mandriva in the distant past). _James
