You are ignoring all the positional 3D code, the effects that go into each voice (such as distance, reverb, materials, EAX 2.0 - 5.0 HD, and the game code loop + DirectX offloading required to process audio).
Audio uses more than 10% of peoples CPU power at times, and when there is heaps going on a 'soft sound card' (eg: a chip that then re-offloads back to the CPU) really hurts performance. (Heck, the game code pushes it via DirectX using Audio Engine, then DX pushes to the Audio chip [hardware acceleration], which pushes it back to the CPU [what I call hardware deceleration]).
Considering how much CPU power this can use, (not 'on average', but 'during re-creatable worst case scenarios, eg: heavy action in an online FPS') gamers really need an Audigy 2 at the least, if not an X-Fi XtremeMusic (but nothing more costly, as they all provide equal audio processing performance).