or you could:
1. use DVD Decrypter in IFO mode, Stream Processing
2. enable Stream Processing in the checkbox
3. select the Audio track (usually 6ch is main movie)and select Raw
4.Convert the AC3 to wav with a program like HeadAC3he or BeSweet (grab the BeSweet Gui too)
the above method does not require ripping the video during the AC3 extraction.
Like Stoner stated, You'll probably have to increase the volume of the wav file.
There are multiple ways to extract the AC3
so you will probably get several other opinions