Because obviously too much density gives terrible sound quality, like in water. So is the answer simply the density of our lower atmosphere because that is where our ears evolved? Or has this been tested and found to be a specific density?
So, how does lower air density affect a loudspeaker's high-frequency behavior?
Primarily, it lowers the efficiency of any pistonic diaphragm. According to the third edition of Martin Colloms's High Performance Loudspeakers (Pentech Press, 1985), the reference efficiency of a pistonic diaphragm (p.27) is directly proportional to the air density and inversely proportional to the square of the moving mass, which includes that of the drive-unit (diaphragm and coil) and the reactive mass of the air load on its diaphragm. The effect of the air density will therefore depend on the drive-unit's design: if it has a high-mass diaphragm, any changes in the air load due to the lower density will be negligible, and the change in efficiency will be directly related to the change in air density. Continues...