Thanks. So is .currency() mainly for non-editable formatting?

Ahh, I see. ReturnsValue is generic and you have to provide the concrete type you are going to return. https://developer.apple.com/forums/thread/723455 So in my case, the perform method signature needs to be func perform() async throws -> some IntentResult & ReturnsValue<Double> The docs could be a bit clearer here IMHO.

After some great detective work from another developer it seems that the issue is caused by the "Play user interface sound effects" option in the 'Sound Effects' section of the 'Sound' System Preferences pane. I had to turn that option off, delete my app, reboot, then re-enable that option and the custom sounds now work. The .caf files still don't work, but that can be worked around.

I've updated the sample app with multiple sounds, in multiple formats. None of them appears to work. Except one MP3 (very odd) - note not any MP3, just this one.

As this appears to be a bug (to me), I have filed a Radar. (FB11642483)

FYI, this is on Monterey 12.6

I cross-posted to StackOverflow and got this useful answer: https://stackoverflow.com/a/68397848/568104 Apparently iBeacon APIs are not available on MacOS, despite the docs stating that they are available on MacOS.