I've worked with the HC-05 Bluetooth module which is a popular module having standard UART TX/RX lines that can be directly connected to the RPi's GPIO. You can use it to set up a Bluetooth terminal without any modifications soft- or hardware wise.
It seems I can set it to announce itself as an audio sink if I change its class which is possible according to the datasheet (AT+CLASS=?). I don't quite know what data it is, some kind on PCM according to the Bluetooth specification.
The idea would be to output music from phone to Raspberry jack via Bluetooth. I don't know yet if chaning the Bluetooth module's class is enough for the phone to recognize it as an audio output source (A2DP).
Are there major obvious problems with this idea? Does anybody know of similar projects? How would one proceed to send data from a GPIO pin to some audio decompressing service?