Old app note from Maxim Integrated on DIY portable bluetooth audio system. Link (PDF!) here
When people want portable music, they usually rely on battery-powered audio devices. With a bit of engineering blood (or curiosity) running in your veins, it is not difficult to build a wireless Bluetooth® stereo audio system that can be controlled with any device that has a Bluetooth connection and a music player.
Can this be used to digitally record the incoming bluetooth audio stream (a2dp?)
Did you actually look into the linked document before typing your question? :)