if anyone is looking for interesting things to do with CUDA
I believe CUDA is very well suited to implement a polyphonic and multi-timbral audio synthesizer on a graphics card (a mixture of wavetable synthesizer and FM or physical modeling). This could include effects as well. In essence the entire process of audio synthesis, track effects (e.g. distortion), mixing and post processing effects (e.g. hall and reverb) could be offloaded from the CPU to the GPU.
So the graphics card in essence becomes something like an integrated wavetable synthesizer, like the dated Yamaha SW 1000 XG PCI card. The CPU would only provide some high level control, for example the MIDI processing.
Since the graphics card does not have an audio out (except possibly HDMI audio?) , the CPU also has to grab the resulting PCM samples and feed them to the sound card.