Short answer: No.
Long answer: I'm sure it's against the regs.
Longer answer: It really depends on what you're doing. I've listened to an iPod (through a VERY not-legal modification to the ICS system) on a cross country with no passengers on board. Would I do it on a mission? No. Would I do it on a flight that had passengers? No. You just can't be a jackass about it.
I'll piggyback on this as I've done similar. It's not the best idea as there's a possibility the music can mask certain sounds. For instance, if you've flown a model a/c for long enough, you know what noises are normal and abnormal. Sometimes it's very subtle. Especially in a helo.