By Kasper Skårhøj @ Ken Stone
This article describes how I managed to create a setup with my Macbook Pro to mix and preview 5.1 surround sound in Soundtrack and Final Cut Pro.
Your mac is able to output an encoded Dolby AC3 stream to a surround amplifier through the audio jack. This works only with DVD-movies and QuickTime movies which already have the 5+1 channels encoded in AC3. When you mix surround sound the 5+1 channels are not encoded (and probably can't be encoded on-the-fly). So you need an external surround sound card.