External sound card, stereo sound to all speakers
Aggregate Device

You can setup an Aggregate Device (Audio Midi Setup > Create Aggregate Device), which allows the ability to combine multiple devices of the same model, or to combine multiple inputs and outputs for apps that don't support separate input and output devices. This Apple guide shows how it works and is surprisingly quite easy to setup. 

SoundFlower

Another way to route audio to multiple channels and outputs (up to 64) is with the free app/plug-in Soundflower. You can download a ready-to-use version, or compile the source code yourself if you have the desire to specifically modify the application. Soundflower is really excellent for many things — what is up to you.