Thanks for the help, but I don't think that would work for what I want to do.
Let me explain my situation. I have a video camera connected to my iMac via firewire, and that is going into a video surveillance app called EvoCam. In this I have it set up to play a sound when it detects motion. It is pointing at my lawn, and I want to use it to scare away foxes from eating my pet chickens. I want the sound from EvoCam to go to speakers out of the line out on my iMac. Meanwhile I want everything else to go through my Bose speakers (which are connected via USB).