Yes, an Event. But to clarify, by "returns" do you mean that the event passes a single boolean parameter? That would be good...I just want to be sure you don't mean a function that returns a boolean value when called?
However, FULL DISCLOSURE:
I have to tell you that the AudioExoPlayer will be running in an Itoo service/process (;which I have implemented).
BUT, earlier I reported what appears to be a bug (Open Source • Background Tasks: Itoo 🚀 - #1165 by Randal_Andress ) that prevents the PhoneCall component's events IncomingCallAnswered and PhoneCallEnded from working...
If you are using these PhoneCall events in your extension to provide this new event, I'm concerned that it may not work from within Itoo either. If you build it, I am happy to try it, but I would not want you to go to a lot of trouble building it when it may not work from Itoo 
... and if it is fixed, then I can simply use the PhoneCall component for these events 
... I'd like to give your new event a try, but it may not solve my problem.
-Randal