I couldn't get an mp3 to play when I first booted BackTrack 5 in Virtual Box (which is the only audio file that I've tried to play so far with BackTack)
I tried installing banshee then the Ubuntu restricted extras since they have the mp3 codec that seemed to be missing. This let me play the mp3 but there was alot of noise/cracking in the background, then I tried the pulse audio fix and that seemed to get it to play smoothly.
I'm fairly new to both Linux and BackTrack so I'm sure that someone will tell me that was a stupid thing to to but it worked for me, here's the code for the command prompt
Then for the pluse audio:
apt-get install banshee
apt-get install ubuntu-restricted-extras
Don't know if that will help you guys or not but did the trick for me.