A VOB (Video Object) file is a container for audio, video, subtitles and other content, normally used in DVDs. Here are two of the many possible ways to extract a song or other segment of audio from a VOB file and put it on a CD in MP3 format.
Manual, GUI procedure: you can use Avidemux to view the DVD and manually save as a file only the video segment containing the desired audio. Then open that file with the Audacity audio editor and recorder to extract the audio track, clip it as needed and save it.
Command line procedure: check when, exactly the audio you need starts and how long it lasts (the example command below assumes that the interesting part starts after 7 minutes and 21 seconds, and that it lasts 2 minutes and 34 seconds). Then launch ffmpeg program to load the VOB file and convert its audio to .wav (in the example) or mp3 format as with only one command:
ffmpeg -i VTS_01_3.vob -ss 00:07:21.000 -t 00:02:34.000 -vn -acodec pcm_s16le -ar 44100 -ac 2 song.wav
this is the synthesis of a discussion on the Ubuntu users list.
Please add suggestions and alternatives in the comments or send them to mfioretti, @:nexaima dot net.