Does altitude change the sound of speakers??

Yes, it makes them sound higher by about 4,500 feet. Lower efficiency speakers suddenly become higher efficiency. A Magnepan can be driven by a 3 Watt amp whereas in Ohio one needs 200 Wpc.  ;) 

Other than that, I suspect that changing a pair of interconnects has more impact than elevation. 

To find your answers to these and other pressing questions, read Mary Roach, author of Packing for Mars, and other scientifically tangential books. 

glupson, don't judge a Roach by the cover. Not nearly as one-dimensional as you think. 

I almost suggested the OP bring the listening room along for a comparison. That's one advantage of making a cargo container your listening room; you can bring it along! Would hate to set up a rig on the long wall, though. 

OP could 3-D print a copy of old room to compare. 

Lots of science to be conducted for the benefit of humanity! 

I'll tell you what's no joke about being 1 mile up, running. Wow, that sucked, as in sucked wind, when I ran whenever I was in Denver. Incredible how the lungs burn from the elevation change. The other time that my lungs burned when working hard was without much acclimatization at Machu Picchu climbing from the complex to the Sun Gate and returning in one hour.

They could make low altitude drivers with holes in them to match the atmospheric pressure's effect on the cone. This is a genius idea and should get me widespread recognition as a futurist, because they will do that in 10 years. If you happen to live in Cleveland and buy the speaker with holes, then move to Prescott, you just put tape over the holes. If you live in Prescott and buy the one without holes, you just use a sharpened pencil to make them when you move to lower elevation. If you have a ceramic driver, too bad. This is straightforward audio, the way it should be done.  ;)

glupson, she is a rather entertaining author. One of the best was entitled, "Stiff: The Curious Lives of Cadavers," I believe. I was reading it when flying over the Atlantic, when a flight attendant said, "What are you reading?" I shared the tile, and she said, "Oh, I read that, too!" 

I commented that I was currently at the part about airline disasters and what happens to the body when a plane breaks apart at high altitude. Didn't bother me excessively, though midair.   :) 

The OP obtained excellent guidance from me, and with humor. 
-New room, all comparisons are relative
-Change IC's, bigger difference 


Now how about a reality check? 
Attendees of RMAF all know how horrid the rooms sounded due to the elevation. The bitching was constant, the frustration endless. Well, actually, not. Actually, no one discusses elevation and effects on speakers, because it is a non-issue. Anyone wants to waste their life on things like burn in and elevation, feel free. :(