Re: How different is Mach 1 at sea level compared to 30,000 ft?

Yes, it is. The speed of sound depends, in general terms, on how easy it is for neighboring molecules to hit one another. They can hit one another more easily if they are closer together, which is the same thing as saying the air is more dense. You can change the density of air by compressing it, letting it expand out of a container, by heating it or by cooling it. At 30,000 ft, the speed of sound is roughly 995 ft/s. At sea-level, the speed of sound is about 1,116 ft/s. There's a Java applet on the web that can do these calculations for you. Fluid Flow Dynamics
Some graphs that may give you an even better understanding are at Atmospheric Data Graphs (PDF file)