Easiest way is to use a program like AudioHijack (I presume you're using a mac), which can intercept (for want of a better word) any audio that's being played on your mac and save it to a file. You can then import the audio into iTunes and sync with your iPod.
There are some command line tools available, but I've not had much success with them
You'll know that DVD-A is stored as encrypted data on the disc,and thus you'll be unable to extract the full 24/96 or 24/192 tracks from the disc....the best you may get is CD quality(i.e.16/44.1) from the digital out of your player/transport prior to transfer to the iPod.
DVD-V is a different matter,and there are programs which will do this,although the issue of copyright may arise.