Here is a Bluetooth sound system that you can fit in your helmet to enjoy your music during long rides. The Syphon Soundwrap has a soft, flexible design. It has 12 hours of battery life. You get 2 flexible loudspeakers, a sweat-proof sleeve, and 2 velcro strips.
Soundwrap is only 1/10″ thin. It can fit into helmets, beanies, and hats. It costs under $200.