Please add some info about the .avi files
I assume the video stream is XviD, but what about the audio stream? MP3? AC3? DTS? AAC? OGG?
Can you hear the audio while playing the .avi clip in Windows Media Player? (try it)
My only guess would be that the DVD maker didn't recognize the audio stream and couldn't convert it to a DVD-compliant format. Do you have a codec pack installed?