get VirtualDUb here:

http://www.geocities.com/virtualdub/

extract the zip file into a folder on your hard drive, and startup the "virtualdub.exe" file.

Now in the virtual dub program, open up your .avi or .mpg file using "FILE>Open Video File..."(virtual dub can read both, but may have problems opening up corrupted avis or mpgs).

When you opened up your video file, goto "FILE>Save WAV..." and set a file name & directory of where you wanna save your .wav file, then click SAVE. Now you should see a small progress window showing how much of the audio is saving. NOTE: when setting the file name, make sure you add .wav to it

Once the saving is done, voila! The outputted file is an *exact* audio extraction from your video.