Best answer (Chosen by asker)
This is usually caused by the compression on the audio of the source file. If it is an AVI file it is very common. With an AVI file the easiest things to do here are set DirectShow Multimedia File Reader's priority to 2 or even better demcompress the audio!
To change the priority of DirectShow, this is dont simply by clicking option - environmental setting. Click the VFAPI plug-in tab. You will see DirectShow in the list usually with a priority of -1. Right click on the NAME (not the priority) and click higher priority until the priority is 2 and it is at the top of the list. To decompress the audio for better results, do the following with VirtualDUB...
1. Click Video - Direct Stream Copy.
2. Click Audio - Full Processing Mode.
3. Click Audio - Compression. Select <No compression (PCM)> and click ok!
4. Click File - Save AVI and try it with TMPGEnc.
