I cannot sing their praises enough. SoundFlower is a fantastic OSX app, and for the AV/VJ community, it is an absolute necessity.
It allows you to pass audio output. back into the internal audio mix with 64 internal virtual channels.
Soundflower is a Mac OS X kernel extension that enables audio to be passed between applications. It acts as a virtual audio device, with both inputs and outputs. With it, any audio application can send and receive audio. Initial support for the development and maintenance of Soundflower was provided by Cycling '74. Stewardship has now been passed to Rogue Amoeba, and we're providing hosting of the latest version. Soundflower is an open source project. Code for the Soundflower project is now hosted on GitHub. Download Soundflower
Let me know how you use it!