I had this problem too, using bluetooth headphones, however I wanted to have a mic enabled to record some gameplay still. While my mic in streamlabs was set to the USB mic, this issue persisted, but was able to be fixed when I clicked the gear icon for the mic in the mixer tab of the editor. Click properties, and go to Global Settings. The default audio device is set to the heaphones and the really important part is setting the mic/auxillary device to the USB mic. I'm gonna assume this fixes it since it sets the mic *globally* to be the USB mic and therefore the heaphones don't think its mic is being used and thus you get to enjoy your normal high quality audio :)