You need to extract the data (which is encrypted) on the DVD. Demultiplex the audio. Convert it to WAV or whichever format you prefer, then copy it onto your portable music player.

This program will allow you to copy decrypted data from your DVD disc and extract the audio streams.
http://www.videohelp.com/tools?tool=DVD_Decrypter

After you get the audio you want, you'll need to convert the audio. Most DVD audio streams use PCM (no conversion needed), Dolby AC-3, or MPEG2 audio.
I would suggest using Besweet:
http://www.videohelp.com/tools?tool=BeSweet