For a while Apple's Mail was totally silent on my Mac Mini Intel Core Duo 2 and I couldn't figure out why. Quicktime, web pages and everything else were fine, it was only Mail. (I don't use iChat so don't know about it.) Turns out I had been using Audacity and Soundflower and I suspect one or a combination of these is what caused the problem.

Trouble is, none of the other web tips for restoring sound worked, such as playing something in Garage Band, setting Audio Output format to 44.100 KHz, and I had already tried the one that fixed an Audacity problem posted above.

What finally worked for me was to use the Audio MIDI Setup application to change the output setting format from 2-ch-24bit to 2-ch-16bit and then back to 2-ch-24bit. After doing that I Quit Mail and restarted it and everything was back to normal. It could have been that changing the other format option of 44.100 KHz to something else and back would have also done the trick. If you are having the no sound problems try fiddling with these parameters.