Take a look at the open-source WavTap, which is a simplified fork of the open-source SoundFlower virtual sound card driver. It is a .kext that I believe does substantially what you want. For reference, here is how some popular commercial closed-source options work: Rogue Amoeba's Audio Hijack Pro -Captures system audio via code based off of the open-source SoundFlower .kext -Captures an applicatio