scratch that, car needs to have AMI for my suggestion to work.
Someone over on Audiworld who is US based got it working - http://www.audiworld.com/forums/a6-s6-c6-platform-discussion-58/diy-streaming-bluetooth-music-via-mmi-2791080/
I know this thread is old, but I thought I’d chime in as I’ve recently been playing with an aux in for my European 2006 S6. I had bought an XCarLink but couldn’t get power to it, and in the end realised my unit was faulty.
In the end I bought an Audi AMI system through eBay (4E0035785B). They’re expensive, and don’t come up very often. I paid £300 second hand, but figure I can resell it and make most of the money back when I sell the car.
I have left the 6 CD changer in the glove-box, but have disconnected it. I used the cables with the AMI and have hidden the AMI behind the glove-box. I updated my MMI software and the AMI works fine. I’ve stuck a USB in with all my music and I can navigate through the USB with the MMI screen. There is a very comprehensive guide to this on a6retrofits. This doesn’t give you Bluetooth streaming but is perfect for me.
I may end up going this route, I just haven’t had the time to try. But I’d like to know if you can just connect a bluetooth 30-pin iphone adapter and have easy, reliable bluetooth streaming. something like this:
I don’t know for sure as I haven’t used a Bluetooth streamer. But I don’t see why it wouldn’t work. You’d get the Audi AMI module, then (from eBay) the AMI to iPhone connector and then connect this to your Bluetooth. Then you’d control the music through the phone rather than through the AMI. However the AMI is quite clever, and it may be looking for an iPhone and not recognise the Bluetooth receiver.
It’s a lot of money on a gamble unless someone else has actually tried it.
cjwalker - are you listening? 
I thought I was…did I misread something?? 
So previous owner of mine bought AMI module and off that i have the Tune2air bluetooth dongle and stream music to that . you can control the music via the mmi controls but no track listing. If i want to see track listing on the MMI display i need to put music on an ipod/iphone or else a usb stick or hard drive
So it works! Does the BT pairing work reliably? Can you leave it in the glove and get a decent connection? I don’t need to see track info - the phone is often mounted above the MMI when I’m driving long distances anyways.
I tried to do a similar thing a few years back connected to a Dennison ICE-Link, but I always had to unplug/reset the BT dongle to get it to pair. I assumed it had to something to do with the way the power was routed to the dongle.
Yes it works fine, some days the BT dongle craps out and doesn’t pair straight away, usually just a reseat of this will fix it