This one is a bit strange. First, hardware specs: Mac Pro Mid 2010 12-core w/ 16GB RAM running 10.11.6 Bluetooth headset (https://www.amazon.com/gp/product/B017H4LSN4/ref=oh_aui_search_detailpage?ie=UTF8&psc=1) While playing YouTube videos, only mono output, not stereo. However, if I open Sound Preferences, sound magically changes from Mono to Stereo, without modifying any settings. Anyone have a suggestion? It only started doing this a few weeks ago.