Getting the audio from the end credits might take a few steps. What I would do would be to get just the end credits as a separate file by using DVDShrink in reauthor mode and manually select the section you want. That would work if you are cutting it out of the dvd. Convert to mpeg to import to TMPGenc.
If you are cutting it out of the mpeg2 file you've already made, I'd use VideoReDo. There is a free, fully functional trial of VideoRedo available for download. VideoReDo also works with dvd files (vobs) but you have to combine them into one stream.
Next step would be to demux the resulting file---separating the video and audio streams. This can be done pretty easily with TMPGenc. Go to File, Mpeg Tools, Simple Demux.
The result would be an audio file, probably in ac3 or pcm format which you can now convert to wma. I never work with wma files though, so you're on your own for a converter. Look over at videohelp.com on conversion tools. I'm pretty sure SUPER will do this type of conversion, but I've never done it myself.
Here are a few links:
DVDShrink and guides
The SUPER website is one of the most annoying on the net, but you will get to a download site eventually.
With the exception of VideoReDo, all of these are free.
The first step, cutting the video out, might not be necessary if the end credits are already in a file of their own, which is quite possible.