I recommend sulfur hexafluoride for something harder to replicate. Nothing like making hackers risk their life to impersonate you

Excellent idea!

Do you need to calibrate it to be able to repeat it, and does that calibration change if you are at a different altitude and in different conditions, such as humidity?

Does merely changing altitude (or ambient pressure) change voice enough to be considered different by a recognition or synthesizing system?

Source: it came to me in a dream.

Despite popular belief, even heavy smoking does not alter your voice in a significant way.

I guess you don't listen to Sinatra.

Depends on what you're smoking