It seems the way MPEGS work is that audio and video is mixed, or muxed, as I belive the term is..
Anyway just get bbDemux from versiontracker and you'll be able to turn your mpeg into a mpg and a mpa file, from there you can change the .mpa into what you like and then re-attach it to the video using qt pro by choosing "add' from the edit menu, or pressing opt apple key v
hth guv