Since the speed of sound depends on the air, which changes at different altitudes, are “Mach” speeds dependent on the altitude? Or is it basically just like 100 °C is the same even at altitudes where water boils at a lower temperature (in that it is a fixed speed based on the speed of sound at sea level)?
In essence? Yes.
Technically, it’s other factors- temperature, pressure, humidity, and whatever else it’s actually composed of. But all of those are affected by altitude.
Mostly? It’s temperature and composition that affect it (and if air was an ideal gas, pressure would have no effect. Open air is not an ideal gas, however.)