Per application volume settings: possible on Mac OS X Snow Leopard?

Although it doesn't work in Snow Leopard there's a nice open source app, BackgroundMusic, which provides for per application volume control in OSX10.10+:

Jack is a downloadable and free software that enables this feature. Originally written for Linux, it is now available for Mac.

This is not possible (like linux does) under OS X. However, there are a few alternatives you can try. One would be to reroute certain apps to “null” so you don’t hear them, or another sound output that has a lower level. All this is possible with Jack, a small free utility.

What I did was basically open the New Mail sound file with an audio editor, and drastically lower its level. :) (Yeah, it’s lame but it works!)