Live duets in a karaoke app with an audience

Your company runs a karaoke app. A singer performs over a song's backing track, and up to 5,000 people listen live in the singer's room. The next feature is live duets: two singers in different homes, often in different countries, sing one song together for the room.

Singers lose the beat when what they hear reaches them more than about 25 ms late. Between two singers' homes, audio takes 40 to 150 ms one way, and jitter adds up to 30 ms more. Listeners are happy to hear the performance up to 5 seconds after it is sung. Each singer uses a phone with wired headphones.

Design how audio flows between the two singers and the room.