Hey,
I have a movie with the audio that's AC3 encoded and I load it into VirtualDub and ask it to Save WAV to open up with the AVS script in TMPGEnc.
The error is:

"The requested audio compression is not compatible with the input format. Check that the sampling rate and channel count of the input match those of the requested format."

What do I do?

Run over to http://www.doom9.org, look in The Guides->Audio Related section, and read the guide for AC3 to MP2 conversion with BeSweet. All VirutalSub has done with it is extract the AC3 from the divx file and saved it off separately. BeSweet will do all the grunt work of getting it into a compliant mp2 44.1KHz .wav file.

Note... I haven't actually done this myself yet, so I'm just assuming that it all works. I just learned this last night.

Thanks. But before I can covert the AC3 audio to MP2, I need to rip it off of the AVI file. How do I do that?

Like I said, this is that part that VirutalDub is doing for you. Just select Audio->Direct Stream Copy, then select File->Save WAV. It'll dump the audio as a ac3 file suitable for the BeSweet process.

Alright I got it now. I tried it and it worked beautifully. Thanks.