Android broadcast intent for audio playback?

Android broadcast intent for audio playback?

I'm trying to build an Android app that uses a broadcast receiver to
detect when music playback starts, and then offer gesture control to
trigger sending a skip track intent back out.
To begin, I just want to set the receiver up to trigger a notification
when audio playback starts so that I can check this concept will work.
I've declared a service and a receiver in my manifest file, and I've
created a service class that (hopefully) creates a notification via
onCreate().
I've crawled a tonne of documentation though and can't seem to find the
correct intent to listen in for with my receiver. Surely something exists
for this?
Regards.