I have debian wheezy + xfce4 on a 2007 MacBook.
On a fresh install and using the debian live disk, the sound works just fine.
On my relatively new install, there seems to be something conflicting. Sounds used to work, after I installed all my preferred applications, now the speakers are constantly silent. Tried to fiddle with alsamixer, no luck.
Only alsa related change, I added this to alsa-base.conf
options snd-hda-intel model=macbook
but even removing this did not help.
How do I debug this (short of reinstalling, but I'd rather avoid that)?
This is probably more a distro question than an Xfce question, you might get a better answer on your distro's forum. It is probably PulseAudio configuration (if PA is used), look at the PulseAudio Volume Control.