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I have a piece of beautiful music in an MP4 video file, so good that I wanted to loop it for this whole afternoon. With Mplayer, I could single cycle this file with an option -loop 0. Moreover, I wanted to play only the sound without the video, so I searched the 'man page' of mplayer and found this:

mplayer -loop 0 -novideo filename.mp4

And voilà!

PS. I also found this post very useful if you want to extract the audio from an mp4 file to an mp3 file.