Does altitude change the sound of speakers??

No, actually, like Vegas, all the rooms sound awful because of the shared AC power and seismic vibrations. Come on, guys! Besides too many 200 lb resonators of the human Variety in the room. 😬

Heh Chorus, I met Mark L. at a Boston audio store years ago.

I moved from 800' to 7220' about a dozen years ago, and do not recall any significant change in speaker sound due to altitude. The ability to listen at volume levels that do a system justice was an improvement very much more readily noticeable.

I do have a premium system in my truck and I experience substantial altitude changes in my travels. What I notice is that my ears plug up and things overall don't sound right. After I clear them, everything sounds much better. Meaning that there are other factors involved and I can't discern any change in the sound of the speakers directly due to attribute.

Humidity also a factor. 

The effects of gravity are reduced at higher altitudes. Thus, it is slightly possible that a speaker cone will behave slightly different at high altitudes compared to sea level. It would not be audible.