Thanks to the heros on the Mactel list I have sound again. If you use Linux on a MacBook of the third generation and your soundcard has been detected but just no sound will come out, putting the following in /etc/modprobe.conf (on Gentoo: /etc/modprobe.d/alsa and modules-update afterwards) should fix the problem. Please reboot or reload snd-hda-intel after changing the configuration. NOTE: it has been reported to work fine on MacBook Pro’s too.
options snd-hda-intel index=0 model=mbp3