I wasn't having any luck getting MC to work over Airplay from my iMac, despite Airplay working fine in iTunes. After reading quite a few messages about using Airplay with MC, it seemed that I would either need to buy additional software or just forget about it.
But just out of curiosity, I went into Playback Options, and under the Device Settings for Core Audio, clicked to clear the checkmark next to "Open device with exclusive access". I then selected my Samsung wireless speaker as the Airplay device in System Preferences > Sound and clicked Play. To my great surprise, music from MC 23 was now playing over Airplay to the speaker. Problem solved!
Could it really be that simple? It appears so, as I changed the same settings on the Mac mini and now it too would play from MC via Airplay, something it had refused to do previously.
Is this strictly a fluke? Is there some reason that "exclusive access" appears to be set On by default? In any case I am now quite happy to be able to use this, even if it does mean less than the highest fidelity.
Hardware:
iMac 27" 5K (late 2014)
Mac mini (late 2012)
Samsung DA-E751 Wireless Speaker
Both Macs running OSX 10.13.3 and MC 23.0.83