After Updating to High Sierra, my bluetooth headphones have terrible sound quality

Solution 1:

Facing exactly the same problem, repairing/rebooting won't work.

But after I switched the mic setting back to Internal Microphone the standard audio quality is back. I guess the audio quality is degraded while the mic is active.

Solution 2:

one option is to enter the following command to the terminal

defaults write com.apple.BluetoothAudioAgent "Apple Bitpool Min (editable)" -int 40

The number 40 might not work for you, try working your way up by running the command with 40, 50, 60 etc. and re-connecting your headset after every time running the command. My JayBird BlueBuds X started working normally at 60.

(https://lifehacker.com/fix-your-bluetooth-audio-in-yosemite-with-this-terminal-1670380974)

Another option that has worked for some people is to set the input device to the internal microphone in Audio settings

(https://stackoverflow.com/questions/37901761/bluethooth-headphone-music-quality-deteriorates-when-launching-ios-simulator)