I’ve done this since college and tried a variety of ways. I do this just for my own enjoyment and because I usually don’t have time to rewatch the film with commentary. I still put commentaries on my digital audio player for listening on the go. To be honest I prefer listening to them without the film.

I like to extract the file (usually ac3) without converting in program and will then convert to flac for playback on my device.

For dvd the easiest thing is to get the dvd audio extractor program. It works like a charm and will let you directly extract an audio track or convert to different formats.

Blu-ray is much more convoluted. It took me ages to figure out but essentially due to the nature of the format you have to go about making a backup as if you were going to add a film to your own personal media server-then once you’ve done that you will be able to extract the audio commentary file itself before deleting said backup. It’s a bit time consuming and frustrating just to put the commentary on your portable player. You also have to deal with some tracks being hi-res which doesn’t work on most portable players so it will require down conversion in that instance. Useac3to is the most helpful bit of software in the world for extracting audio from various files.