You have some music playing, and you're in different room to the speakers, or you're doing chores which generate a little noise, like vacuum cleaning or washing dishes, and you can't quite hear the music properly, but what you csn make out sounds completely different to the song that's playing, and what you're hearing gives you a great idea for a song or section. Does that happen to anybody?
It's happened to me quite a lot, but I've never got around to doing anything with it. I'm always convinced it was a great idea, but I've never been able to put it to the test.
I write most of my riffs to mimic the noise of a vacuum cleaner. Brutal.
Seriously though, elaborate, don't quite get the gist from the OP. The music playing is someone else's and you have an idea for your own music while it is playing? I'm sure that happens to everybody.