Some Bluetooth devices use a lower-quality audio method when acting as both a mic and a speaker or headphones.
Audio quality via a Bluetooth device can be a great mystery, partly because Bluetooth only mandates in its basic audio standard, A2DP (Advanced Audio Distribution Profile), a low-quality audio encoding algorithm. An audio encoder takes a data stream and transforms it into whatever the best or agreed-upon quality both sides can take, often limited by the particular chips on each device and bandwidth available.
That means that the lowest-common denominator may prevail, even when both the sending and receiving devices have a variety of higher-quality options available that don’t match up or aren’t correctly invoked.
Depending on a host of factors, both a Bluetooth streaming device (sending or receiving audio) and a Mac may agree on an encoder that makes music sound like it’s playing over a landline telephone call.
Read more at MacWorld.com