A one-way chain of collaborators.
The further you are along the chain, the more of the music you can hear.
If you are first, you can only hear yourself.
The second collaborator (b) receives the signal from (a),
mutes their own feed and sings the following melody on top, recording/transmitting it through another programme;
in this case Photo Booth.
The audience can hear* the combined result:
((a) + b)
* the balance is not great in this recording, but that is not a fundamental issue with the network so much as a mediocre recording setup. We will upload a clearer example soon.