[Ed Zachary]

+1 what brif4i said.

If the phone sees the Prius as phone and music headset, you can play music on the phone and hear it over the car radio.
The key is in setting up the radio to see the phone as a player.

The radio's setup for handsfree phone (that you have already done) is probably separate from its setup for bluetooth audio player (that's what it is probably called in your owner's manual).