I only listened to music twice on a flight deck.
First time was many moons ago on an Airbus flight deck, operated by a charter Mexican airline. I was dead heading with them and went up front for a chat with the pilots, being acquainted with the F/O at the time. To my surprise they were listening some nice classical music on a portable CD-player and it remained on until we parked at the gate, albeit below 10.000ft on a very subtle volume. Didn't notice any disturbance that this might have caused and the PIC didn't pitch or roll the aircraft following the music's rhythm.
Second time was a couple of months ago while en route on a long x-country flight from Sao Paulo to Porto Seguro, on a Cirrus SR-22.
It has a system that allows an iPod/MP3 player to be connected via a line-in jack and the music is automatically suppressed anytime the ATC the frequency is busy. We were VMC all the time and it did not disturb me at all. Would not do it IMC or in busy airspace, though
As mentioned before, rules are for the guidance of wise...
Cheers
BF