I recently tried to watch a video on my Intel Mac where the audio wouldn't work. On the PC I typically just grab k-lite, a large collection of codecs, and that takes care of the problem. Unfortunately I wasn't aware of such a thing for the Mac. Dean Saxe came to my rescue and pointed out this great resource:
This was just Digged yesterday but I must have missed it. (Spent the day in New Orleans.)
Anyway - I downloaded the package and it worked perfectly. My 100% legal/not ripped movie now works great.