There's an assortment of codecs that can be used by the Bluetooth A2DP stack, the protocol is extensible but the only required codec is SBC. Apple supports AAC

There's an assortment of codecs that can be used by the Bluetooth A2DP stack, the protocol is extensible but the only required codec is SBC. Apple supports AAC and Samsung backs APTX so that doesn't help. You usually end up with the lowest common denominator. I've tried out a number of in-car bluetooth setups over the years and I've found that the quality varies pretty significantly between both auto makers and aftermarket units. Ford and Toyota are pretty solid where as Chrysler and Honda are pretty awful. As far as aftermarket solutions go, GROM is decent and USA SPEC is horrid steaming crap. Another unfortunate occurrence I encountered was that Android paired with certain systems (Honda) will prefer to stream audio over the HSP/HFP stacks instead of A2DP or will switch when a call is incoming and stick. This is tons of fun because HSP/HFP use very low quality codecs meant tuned for voice applications. Now that Bluetooth is high bandwidth, I would like to see them add a new audio streaming stack that just accepted PCM/FLAC/AAC/MP3/OGG in all their glory.