Simply focusing on the sound and resting one-pointedly on it is shamatha meditation. Recognizing the nature of the sound is vipashyana meditation. Focusing on the sound, looking directly at the nature of the thought apprehending the sound, then letting go and relaxing into the nature of that thought is mahamudra meditation. Resting in a state free from the duality of perceived and perceiver - an object of sound and a thought apprehending - is meditation according to the chittamatra approach. But if you are really skilled, it will become mahamudra meditation. It all depends on your level of skill. If you recognize that sounds, like dreams, are the expressive power of mind's abiding nature, mahamudra-luminosity, and if you look directly at their nature and relax within it, that is mahamudra meditation.
Khenpo Tsultrim Gyamtso Rinpoche, from his commentary on The Profound Inner Reality given at Karme Choling, Vermont, 2000 (p. 154). Translated by Elizabeth Callahan.