Turn the phones in the room into one speaker. Same song, same instant, no lag between them.
If this phone sounds slightly behind the others, drag right until the clicks land together. Every phone takes a different amount of time to push sound out of its speaker, and a bluetooth speaker adds a lot more. The clock cannot see that, so this is the fix for now.
Point a camera at the code, or type the six characters.
Point this phone at the other phone's code.