Turn the phones in the room into one speaker. Same song, same instant, no lag between them.
Measuring…
Offset is how far this phone's own clock sits from the server's, and every phone has a different one - seconds apart is perfectly normal and is corrected before a note is played. The number that says whether the music is actually lined up is off by.
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.