Are y'all talking about BT for both ears or just one?
I use a Plantronics 320 BT (one ear) and I can listen to music through it. Once the song is playing, I go to phone-voicemail-audio and select the BT device. I can hear it through the earpiece and also through the iPhone speaker. (Odd effect) There doesn't seem to be a way to turn off the speaker using this method.
The music fades out when a call comes in. I have to go back to phone-voicemail-audio to select the BT device again once the call is over.