I haven't really shared this outside of my immediate circle, but I actually have a project like this. It plays on ~all events at the moment but has a special multi-tone sound when an agent is done or asking permission.

It's a cli that runs a server, a browser-based UI (where the audio happens), and a set of scripts to install hooks for various harnesses.

https://github.com/neiltron/bingbong

I have no idea, I just asked both of them to add the hook to themselves.

All agents I know of have a hook system that can call arbitrary scripts.