Some programs can convert DVD to MP3 directly.

When I was ripping DVD to MPEG-4 I did the following:
1. Rip DVD using freeware DVD Decrypter (no longer available). When ripping DVD I join .VOB files of the main movie into single file.
2. Demux sound track (usually it was an AC3 audio stream).
3. Convert demuxed sound track to MP3,OGG,... using freeware BeSweet. I liked BeSweet 'coz it can boost audio by compressing dynamic range.


If you wanna extract a song from your movie, you can use direct sound recording software. Such programs can record any sound produced by your sound card.