The ATI sound implementation is not GPGPU code. It is dedicated silicon, probably brought on by the Vista DRM infection and MS twisting arms to force it on people.
In any case, R600 will be compliant with the Vista requirements and can send sound directly over a HDCP/HDMI link. We are told this is a full HD sound setup, not a cheesy 2.1 channel thing.
In contrast, NV G8x parts can't do this. They have to run an external cable from the sound chip to the GPU. This may not sound like much but it blows out several kinds of auto configuration and worse yet violates Vista logo requirements. One has to wonder if this is why NV can't seem to make a functional Vista driver six months in.