Hi!
For nearly three years, a bug has been known to cripple SPDIF output on certain hardware. It might work fine for a while, but will randomly break, showing two symptoms: PCM audio is distorted (similar to clipping) while compressed audio (AC3 or DTS) will not work at all. Muting an unmuting the master channel may temporarily fix it.
This affects at least all Asus P5B and P6B mainboards.
The solution is a simple one-liner, and has been known for a year: http://fjerhammer.dk/ad1988_spdif_crackling.patch (credits go to Javeed Shaikh for the patch and fjerhammer for hosting it). I can verify that it fixes all issues on my setup. Please apply it.
See also: https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/359361
Greetings, Robin Kreis