Sooooo I have a 2015 Macbook Air that had the missing speaker problems…tried all the resets with no luck and everything else. I unplugged the power and the external monitor adapter and did an apple diagnostic boot (hit power button then hold down letter “D”) It went through the diagnostics and came back with a battery problem (its aftermarket and brand new and works great) but when I restarted from the diagnostic screen low and behold the sounds chimed…just passing on what worked for me