I believe that is going to depend on what program you're using to play (decode) your m2v files. So what are you using?
btw, If you can open both video and audio in Quicktime Pro, you can sometimes use Edit->Add Scaled to make the duration of both tracks the same. Just be careful not to 'Add scaled' audio to anything, or you could make everyone sound like Chip 'n Dale... or Barry White...