Does altitude change the sound of speakers??

yes it definitely does change. it's all related with sound velocity. if humidity, density or pressure increases sound velocity in air increases too. sound is lazy. sound waves tend to move to where sound velocity is lower.
there is no way to come to a conclusion with one variable like altitude because humidity has a greater impact on sound velocity. if humidity is the same on both places than higher altitude cause more absorption than lower altitude on high frequencies. and it's perceived as slight decrease in treble and attack. same happens if humidity decreases. 
it's all related with sound propagation and behavior of sound in fluids. air or water more or less same rules apply for sound propagation cause both are fluids. I'm a submariner. and whole submarine warfare and anti submarine warfare (ASW) is based on tactics regarding sound velocity profile (SVP). 

Boomers.

Google: "speed of sound at sea level vs altitude".
While the speed of sound varies with altitude the reference is to a standard temperature.
Humid air is also less dense therefore lower in pressure and changes
the speed factor accordingly.
For a Pilot operating an Aircraft at 30,000 feet Pressure Altitude becomes a life and death matter but a 5,000 foot difference in a listening room will not change the factor enough to make much difference.
However Caribou Ranch Recordings seem to have some notoriety amongst recording artists.

The speed of sound is irrelevant if it’s the same everywhere in the room. Having “temperature gradients“ in the room would affect the sound, though - E.g., Chinese ice water bowls out in front of speakers.

cm, your ears did not plug up. As the pressure changes you have to equalize the pressure on the other side of the ear drum through your Eustachian tubes. Usually a good yawn will do it. Divers know all about it.
Only the speed of sound changes with altitude. Everything else stays relatively the same. In other words there is no difference from a sonic perspective.