How do you record just the sound off a DVD?

Most DVD recorders have Audio and video output sockets on the back.

Use the Left and Right audio out and connect to a recorder.

These days most people will want the audio stored digitally.

You can use a patch lead that combines the two RCA connector to a single stereo 3.5mm jack plug, use the jack plug in the mic socket of your computer and record using Windows recorder or equivalent.

You can also buy dedicated MP3 recorders that accept analogue inputs, or use a capture card or dongle.