If you've got some MP4 videos and want to quickly convert them to say just the audio track, it seems hard to find this. The simplest (if most arcane way) seems to be for iTunes 11:
- Start iTunes 11
- Import your MP4 videos into iTunes
- Switch to the Movies, click on the Home Movies Tab
- Click on the video you want to change
- Choose File/Create New Version
- While you are looking at the greyed item, hold down the Option key
- Automagically, the last item changes to Create AAC Version...
And you can do it. Why, I don't know. What a strange user interface.