[AAR.oner]

i'm assuming by "vid" you mean DVD

first you need to demux, which leaves you with the audio track [ac3, PCM, etc] & video track [m2v] -- MPEGStreamclip is free and does a fine job

next, take the m2v file and the new audio track and re-sync in your DVD authoring program [DVD Studio Pro is what i'm familiar with, guessing you can do this in iDVD as well]...build the new menus, etc. & yer done

PM me if ya need any help