My experience was not that extreme but I’m always open to new challenges
The challenge of the snow / Sun and Wind, probably more than altitude, was staying in tune (not to mention that the tuner had low battery (due to low temperature) and Sun+Snow made it very difficult to read the tuner screen…
And yes, 5300 ft in an uninsulated garage count as extreme
For the salt mine, we were allowed to do so as we celebrated a Mass in one of the chapel.
Fun fact, the security guard at the entrance looked at us, was super surprised to see us with the instruments, probably tried to figure out what was written in the procedure … and let us in without asking questions
As for the sound, nothing specific to mention we stayed in tune and the sound was OK
I don’t have recordings but overall we were more in survival mode
A friend of mine is a bass player and he played in a throw together band on our Guided Missile Cruiser in the middle of the Pacific on the way to the Persian Gulf for during the first Gulf War! I didn’t have time to do any of that except watch them play occasionally!