I've heard the phrase used in two senses as regards music. One is the device or line used to capture the listener's attention. The other is in the nature of the "message" or "punch line." It's easy to think of it as a fishing metaphore. Then there's the older vaudeville notion of the hook - basically a people sized (but non-fatal, at least in the literal sense)gaff hook used to "encourage" some performers off the stage. I'm looking forward to what others might have to say.