Media does not play when the App is in the background on iOS

While trying to trigger a sound to play in the App (running in the background or screen locked) via a button press on an external bluetooth device, I receive the error:
"message":"","code":4

I've also tried creating a new blank app and wrapping the play method in a timeout so that it triggers a short while after I lock the screen or minimise the App. I do not hear a sound until I reopen the App and I don't get any errors in console this way.

I have set the playAudioWhenScreenIsLocked option to true, despite this being true by default.