The main problem simply lies that DVD don't support MP3. It supports MP2 (Mpeg Layer-II, or 'Video-CD Audio' and AC3 'Dolby Surround', 5+1 channels (in the original DVDs) or 1+1 channels (if you create the AC3 from a MP3, which is n 1+1 channels audio already).
A little suggestion:
- extract the sound as an uncompressed WAV from your AVI (just load it with VirtualDub, se Audio___Full processing mode and do File___File WAV)
- convert that WAV into an AC3 192 kbps, 48 kHz sampling rate (and, you're forced to, 2 channels) with FFMPEG GUI.
FFMPEG GUI is very friendly and is 'written' expecialy for DVDs. It cannot create MP3s (I use HeadAC3he, for this) , but creates AC3 and MP2 (both supported by DVDs), changes the sampling rate (usually AVI are 44100 Hz, DVDs are 48000 Hz), uses your desired sampling rate (usually 192 kbps is for AC3 1+1 , 224 kbps is for MP2 (which, too, is an 1+1 ausio stream) and 384/448 kbps for AC3 5+1).
You should consider AC3 as an 'optimal compression' which, as MP3 (which is dedicated, usually, for music and, in that case, is even VBR instead of CBR - which is the mode which must be used for AVIs). MP2 is a 'less effective' compression (and therefore, 224 kbps instead 192 kbps is suggested).
Read http://forums.afterdawn.com/thread_view.cfm/351013 (which no one commented, and that maybe no one ever read it, the same end my FAQ made) if you want to lern what to do, with that AC3 sound stream [no picture? of course! AC3 is an audio stream, and must be authored together a DVD video stream, to create a DVD].
