Ok, I was just making sure, as many people who post about avi files here do not do their own captures. I used to capture to avi (huffyuv) years ago myself, but got lazy and use an mpeg2 capture card these days.
You’ll need to find out the type of audio you have. Examine the avi file with MediaInfo. If it is in a format that is acceptable for dvd-video, all you need to do is extract it. You can do this very easily with AviDemux. You can also convert it with AviDemux if necessary by selecting AC3 (Aften) as the audio codec, set the bitrate to 128 or so, then click on filters to set the resampling to 48000hz.
To save just the audio in AviDemux, click on Audio, then Save Audio. Set a place on the hard drive to save it, and make sure to make a file name with the appropriate extension…for example, Weekend at Bernies’s II.ac3
Now, adding the extra soundtrack while maintaining the existing menu and chapters is the tricky part. If the new file is the exact same length in time as the other sound tracks, you can use it in the new dvd. Unfortunately, I don’t know the process by which you keep the original menu…never done it. I’d just demux the original and rebuild the dvd with a new menu and new chapter points. That is relatively simple.
Keeping the original means using PGCDemux, Muxman (or other authoring tool) and the pro version of DVDRemake. DVDRemake is not a free tool. Here are the instructions for that such as they are: http://www.cdr-zone.com/forum/viewtopic.php?t=9869 The instructions are byzantine to anyone not familiar with DVDRemake!
Edit: Found another thread on this over at videohelp.com Maybe it will help you: http://forum.videohelp.com/threads/301682-Adding-a-second-audio-track-to-the-exiting-DVD-movie
Look at the second post and the guide it links to.