Follow the guide on this site at http://www.vcdhelp.com/dvdtovcd.htm and pick it up from the DVD2AVI part.

You will see how DVD2AVI can split the audio off into a separate PCM (.wav) sound file and provide you with a VFAPI interface for the video so you can load it into TMPGEnc.

The VFAPI interface can be created by clicking 'Save Project' rather than 'Save avi' under the file menu.

Good luck!