There are a number of clever audio processing algorithms implemented on GPUs already. But that's not the issue at hand, which is doing so in a low-latency realtime context. GPUs are insanely powerful, and if you're just doing playback of material from disk, the tiny bit of extra latency they may cause isn't a problem, so go for whatever they can do. But if you're using the computer for synthesis or FX processing, you need better than GPUs can offer in terms of latency (for now).