IPhone 5 won't stream music through Bluetooth?

I have got a Nexus FM transmitter and iPhone 5. The devices pair and I can receive phone calls and hear Siri through my car stereo, but every time I play a song, it comes out of the iPhone speaker.

Does anyone know why this might happen or how I could fix it?

Thanks

Added (1). Constructive answers from people who aren't jealous of iPhone 5 please

Has anyone got a source for iPhones not transmitting music over Bluetooth?

Firstly your paid 500 quid for an extra row of icons lmao
And yeh check the setting in setting on iPhone to allow muic output thru Bluetooth.
Also may need to check it on iPod tunes what

It sounds like the Nexus FM transmitter is not A2DP compatible. Not all Bluetooth devices can transfer audio sources that same way they do phone audio. I know this sounds strange but check the link below. You will need to verify in the manual for the your transmitter if it supports A2DP. Many of the older Bluetooth 2.0 devices do not support this profile setting so they will never transmit stereo audio like you need.