Does it seem feasible to use sound waves to deflect an inbound asteroid?
Would it matter what frequency the sound waves are? Would it matter what the size or mass of the asteroid is?
No, sound waves can't traverse vacuum of space. They would only work in the atmosphere when it's already too late.
Space is almost vacuum so the sound wave will not propagate in the space. Instead, if we talk about the radiations, the impact of photons will impart pressure on the asteroid surface and might help in changing the course of the asteroid. The sun exerts solar pressure on cosmic objects. If we devise a technique of making the asteroid more/less reflecting then by the medium of varying the solar pressure on it we can change its course.