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

Solution 1:

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!)

Solution 2:

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+:

Solution 3:

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