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Zipper: Oh, I see what you were saying in your earlier post! You were 100% correct, and I was wrong. My mistake, sorry. I thought you were saying the speed of sound was the same at every altitude, but that was because I didn't understand what Mach 1 was until this very moment.

I actually thought it was the speed of sound at sea level. How many times have we heard that it was? I know I've heard it many times (including, I think, from some physicists) and just took it at its face value without ever really thinking about it. Mach 1 is a variable that depends on many things but has no meaning unless it's applied to some object or body that's moving through the air (or medium in question). It varies from place to place in the medium, depending on where the moving object is, what the composition, density, temperature and pressure are at that point, etc. You can be flying along at Mach 1 one moment and then the next moment NOT be flying at Mach 1, even though your speed hasn't changed. Thanks for the schooling, guys! :-)