A huge part of the perceived feel of a club is the sound it makes with its strike. If you dampen that sound with headphones, in my opinion you are doing yourself a disservice. I only have seen one Tour Player ever have headphones on during a range session. In college, it was an epidemic, everyone did it.
The metronome app is an interesting idea, have not tried it.
With that being said I do not mind if music is on during play, on an iPhone speaker or such while riding in the cart but certainly never headphones. I was in the middle of a round with a good friend from college and he put on some Club/House music and I looked at him with a, "are you kidding me" kind of look. That is where I draw the line.