Does altitude change the sound of speakers??

Funny you say that. I've actually seen a flight attendant going down in style on a stiff one.

Anyone can talk talk. In the interest of science I have done the work and can now state unequivocally the effect of altitude is positive. The higher I am, the better it sounds.

Actually, it was from the first Alien movie advertised as “in space, no one can hear you scream”. I still have the movie poster.

Yes but my kid brother had the "In space no one can hear you fart" T-shirt. 

Correct.

Sound is pressure waves transmitted by molecules bumping and bouncing off each other. The greater the density the faster and more efficient the wave. Air at sea level being more dense the sound will be louder with more dynamic impact. Go higher, air gets thinner, more tenuous, sound and dynamic detail are less. The difference the first mile is however much less than a dozen other things that change such as for example the room. The room and setup contributes far more to the sound than changes in atmospheric pressure.

To hear a difference greater than all the other differences you need to go higher. At some point high enough density falls to zero, no molecules, no waves, no sound. Thus the expression, In space no one can hear you fart.