Bluetooth streaming is quite poor due to the compression used to transfer the sound over bluetooth. I tested a bluetooth adaptor on the end of my Ipod connection in my A2 and it was noticeably poorer compared to a wired connection. I've not tested the bluetooth transfer in my A7, but I'm guessing it would have the same limitations.
I'm now using the Audi Music Stream app on the Galaxy S3 and it allows browsing of your phones media over wifi, so no compression (or very little) and the sound is superb. Of course this only works with a car that supports it such as the new A6/A7/A8/Q7, I don't know if the new A4/A5/Q5 with HDD navigation, online services & technology facelift supports it as well.
You can also browse internet radio stations using the app through the car. Pretty cool I thought.