After my slightly encouraging reports on AirPlay under Sierra, the newer High Sierra (macOS 10.13) has proven to be a catastrophic step backwards. AirPlay under BitPerfect failed completely. You would go through all the hoops to get BitPerfect to select your preferred AirPlay device as its designated audio output device, but as soon as music started to play - and BitPerfect hogged the AirPlay device - macOS would observe that the device had suddenly disappeared, and would delete its Standard Audio Interface. Since this is what BitPerfect was using for playback, it meant that playback instantly stopped, before it even got started.
Well, without any fanfare, it seems Apple have fixed this with the 10.13.3 update to High Sierra. I am now getting it to work. The approach is pretty much as described in the previous post (which I linked to above) except for one thing which has changed.
As before, you still need to go into iTunes and in its “Choose which speaker…”
selector (the button with a ‘transmission beacon’ icon to the right of
the iTunes volume slider). But now you need to be sure to choose the AirPlay device that you actually want BitPerfect to play to and NOT "Computer".
I hope this is a sign that Apple are taking seriously the situation where third-party software (such as BitPerfect) "hogs" a device (in Apple-speak, puts it in "exclusive mode").