Strange how TMPEGEnc doesn't extract the sound.
The other way you can do it is to use VirtualDub. This small program can extract the sound into a WAV file and you can Convert your AVI and sound separately but at the same time. i.e when you go to convert in TMPEGEnc, you select the source AVI file, Just beneath it, you can select the source sound file as well.
You can get Virtual dub from here click here