I have multiple A1-MR, but I only see some of them outside the Audioengine App. Where did they go?

If you have grouped the speakers for a multi-room setup in the Audioengine app, only the A1-MR that you grouped the speakers into will show as the available destination. If you wish to send to your other speakers independently, ungroup the A1-MRs in the app by pressing and holding on the speaker in the grouping and dragging down and away from the group.