Since you mentioned FFT, I wondered whether GPU-based computation would be useful for audio DSP. But then, lots of audio DSP operations run perfectly well even on a low-end CPU. Also, as one tries to reduce latency, the buffer size decreases, thus reducing the potential to exploit parallelism, and I'm guessing that going to the GPU and back adds latency. Still, I guess GPU-based audio DSP could be useful for batch processing.