Although atmospheric pressure and density influence the speed of sound, the two effects essentially cancel each other out. In fact, the most noticeable factor affecting the speed of sound at high altitude is temperature. At colder temperatures, the molecules in the air carry less kinetic energy, making sound waves travel more slowly.
At -1 degrees Celsius for instance, sound travels at 330 metres per second, compared to 344 metres per second at 21 degrees Celsius. However, the affect this has on the frequency of sound waves – and therefore their pitch – is so small that music would not sound any different.