IPhone Music app doesnot work with car audio bluetooth system but Google Map works?

I have iPhone 3GS which have google map app, it works fine with car audio bluetooth system. I could here turn by turn direction in car speaker. But why music app does not work the same way?

Any clue.

Added (1). My Car is 2009 Camry LE which have A2DP support

The head unit has to be A2DP capable in order for music to work.

Google map app on my iPhone 5 does not work with car audio Bluetooth system on my camry

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