Ok, the old fashioned way will work if nothing else will. Get DVDFab Passkey (which is still free to use while in beta) to break encryption on any modern commercial dvds you'd like to work with. http://www.dvdfab.com/en/passkey.htm Let it run in the background.
Also get DVDDecrypter, http://www.videohelp.com/tools/DVD_Decrypter the old standby for working with dvds. You can remove the audio stream that you want by using DVDDecrypter in IFO mode.
Start Passkey, then put the dvd in the drive. When you start up DVDDecrypter, go to Tools-->Settings, then remove the check in the box that says Check for Structure Protection. Then click on Ok. You only have to do this once.
In the main window of DVDDecrypter, Click on Mode, then set it to IFO Mode. The main window will change to IFO mode and you will see a window on the right with two tabs. Click on the Stream Processing tab, then put a check mark in the box that says Enable Stream Processing. You will see a list of all the video, audio and subtitle streams. Leave a check mark next to the audio stream you want, but not in anything else.
Now at the bottom of the window you'll see three options. Here you select Demux. Keep the audio stream you want highlighted. Click on the disk-to-hard drive icon on the left to start the process. The output will be a .vob file, but it will only have the audio in it. The vast majority of audio files in dvds sold in the US will have ac3 audio, and if that is what you started with, you can safely change the file name with an .ac3 extension. Edit: Wrong...if you use Demux and highlight the audio file it will come out as an ac3 file so disregard this.
The next program is HeadAC3he. http://www.videohelp.com/tools/HeadAC3he This one is fairly simple. Just import the ac3 file, designate a spot to put the new .wav file and let it work. If you want to change from 48khz to 44.1khz you can do that with this program too, but it is not necessary unless you eventually want to go to cd audio. http://www.videohelp.com/tools/HeadAC3he
Now, why should you go through all this? Believe it or not, it is faster than recording the dvd as it plays. It will also keep from degrading the audio in any way.
Edited a few incorrect things in here.