I had a piece of very good music in an MP4 video file, so good that I'd like to loop it for a whole afternoon.
With mplayer, I could single cycle this file with option
Moreover, I wanted to play only the audio but not the video, so I searched the 'man page' of mplayer and found this:
mplayer -loop 0 -novideo filename.mp4
PS. I also found this post very useful if you want to extract the audio from an mp4 file to mp3 file.