A local guy, here in Tampa, made these and was selling them at local bike shops. They go under your jersey in the area of the collar bone and are held on with magnets. The company went out of business due to poor customer relations and he was unable to supply the demands of the orders.
I purchased a pair from an LBS and they work great. You can hear the music and everything else around you with no problem. You can even carry on a conversation while listening to the music. The speakers are high impedance, so even with the volume turned all the way up, the maximum output level is not loud enough to drown out the surrounding noises. I wear them on every ride.
As you can see, there isn't much to making a set of these, you just need to find the right speakers, which are water resistant, BTW. The magnets you can get almost anywhere and that is just a piece of 3/16" or 1/4" plastic he uses for the base. The pieces are epoxied to the base. Assembled, they ran about $30.00, but I'm sure you can buy the parts for a lot less.