Eh, I can only guess that people put their hands behind their backs so they won't lean on the handlebars and hold their weight up with their arms... thus engaging their abs and core muscles more. I don't get it though. One lady I know who has been going to spin classes for years... I think it's produced a hunched back with her. Or she just has really bad posture.
As for bringing in your own music... I've done that. I won't lie. A couple of years back I had to go to spin classes to get my rides in, as I didn't have a bike trainer at home. The music that motivates me is music that doesn't motivate anyone else... and thus, I always hate the music that is being played in class.
So, I would go to the back of the room, in the corner and take the class from there with my headphones on. I would try to stay away from being in front or anything like that. I just wanted to ENJOY the class without having some horrible music being blared in my ears.
One instructor finally told me that I couldn't wear them because her manager said no more... because it was a liability... if I couldn't hear what she was saying I might get hurt. OH REALLY? Oddly I can run on a treadmill, with headphones and not get hurt... but somehow I'm going injury myself on a stationary bike? I followed up with her manager and found out it was a lie. Which really ticked me off.
Thank goodness I have a bike trainer now and I don't have to go to spin classes anymore.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"