I am using GK to convert some mpeg files to xvid, but just forgot to choose to mix the audio with the encoded xvid file. So now I have an avi wrapped xvid with video only and a separate mpa file in the harddisk. Is there any tool that could help me to mux the audio track back into the xvid or I just have to choose to re-do the GK again on the original mpeg file? thanks.

Virtualdubmod. You need to run the mpa through besweet to convert it to something like mp3 first though. (Besweet will also correct for its delay if you didn't rename it.)

VirtualDubMod, or AviMux

I did it with Nandub. My audio stream is a mpa one (which is mp2 by nature). I just use Nandub to direct stream copy the video part and also mux the audio part. I do not see a mp2 selection in the Nandub audio menu, so I just choose mp3 and it worked and can play ok on my mediaplayer classic. thanks folks.... I did not know those tools are in fact available in my GK folder... :)) until I use them.