Bluetooth's Audio Profile A2DP supports multiple codecs. All devices have to support SBC (subband codec), then they can support additional "optional codecs" like MP3 and AAC, or "non-A2DP" codecs like apt-X.
Of course these codecs can't actually be used if the receiver doesn't also support them, in which case both devices fall back to SBC.
- How do I find out which codecs my hardware/ROM support?
- How do I find out which codec is currently in use? (Maybe this depends on the track, too, if it passes MP3/AAC files directly without re-encoding, for instance)