sound output state of individual apps Linux?

So, /prog/, do you know of any software I could use that might be able to fade my music out when another application begins emitting sound? I'm using Chromium, and Linux with ALSA installed. I could write something myself, but would at least need a tutorial or some pointers on how to read the sound output of every application, which I've realized I have no idea how to do.
I was going to post this to /g/, but they've never been much help in my experience.

That's especially unhelpful when I'm running gentoo. This is why I don't go to /g/, and had hoped /prog/ might actually know something.