mp4UI is an easy-to-use software for opening, creating and modifying MP4 files. It comes with a nice GUI and among others it lets you import, export, delete and hint tracks, edit properties and metadata and view information.
Extremely simple tool to demux an MP4. Runs perfectly under wine 1.3.30.
its so very easy to remove the video from an mp4 (to get a music-only-file) that even my grandmother could do it. Very nice tool!