I had this problem recently. I had downloaded videos from Coursera and I wanted to watch/listen to them on my Ipod Touch. Of course, nothing is straightforward on the idevices. I tend to listen to the lectures in the gym, so mostly listening and not watching. On my Note 2 I can lock the screen so that the video continues playing while I put the phone in my pocket. But the ipod does not. So I needed to extract the audio from the video (fortunately these lectures are not visually intensive).
Handbrake does not do it, and other options I found include installing ffmpeg on the command line using homebrew. Finally someone informed me that I could do this natively in the Mac OS. This page contains the details. The gist is the following:
Encode video service on
Go to ’System Preferences \( \rightarrow \) Keyboard \( \rightarrow \) Shortcuts. Click on Services and ensure that the “Encode selected video files” and “Encode selected audio files” services are selected. This will make these options visible when you right-click on a video file.
Select one or more video files, right-click, Services and select “Encode selected video files”. In the first dropdown at the top, select Audio only. Select where to store the audio files, then encode.
The nice thing about this is that you can batch process.