The situation goes a little something like this:
I am programming Xcode whilst concurrently listening to music on my bluetooth headphones... you know to block out the world.
Then, I go to launch my app in the iOS simulator and BOOM all of a sudden my crystal clear music becomes garbled and super low quality like it is playing in a bathtub 2 blocks away... in the 1940s.
Note: the quality deterioration does NOT occur if I am playing music on my laptop or cinema display and I launch the sim. Seems to be exclusively a Sim -> bluetooth issue.
The problem is more than just annoying. Because often after stopping the simulator the crappy bathtub quality music continues. To fix it I have to open sound preferences in OSX and briefly toggle back to my laptop sound and then back to my bluetooth headphones.
This is a big deal, because I launch the simulator 50x a day and have to do this toggle thing every time as well as suffer listening to 40s era mono ham radio quality music.
For your information, the headphones I am using are Plantronics Back Beat Pro and I am up to date on firmware. I am on OSX 10.11.4 and Xcode 7.3... but this problem has persisted through all versions for 2+ years now. Can you save me from the 1940s?
I've managed to fix it, and it actually seems to be a microphone issue. Go to System Preferences -> Sound, select the Input tab and set Internal Microphone as the input (mine was set with my headphones').
Crappy sound goes way after that =)
EDIT (May 30 2018):
I've found out an easier way to do the same as above. Instead of opening the System Preferences, you can just go to the Mac OSX toolbar, press Option (alt) + click on the sound icon and then select "Internal Microphone" from the "Input Device" list. Print screen as follows.
If you're using Xcode 9 or higher, you can set a default audio input and output for the simulator. This can be done by launching the simulator from Xcode and navigating to Hardware > Audio Input within the menu bar and selecting Internal Microphone. This solution will save your audio preference so you won't have to change it on every launch.
来源：https://stackoverflow.com/questions/37901761/bluetooth-headphone-music-quality-deteriorates-when-launching-ios-simulator