One of the reasons mach is used at higher altitudes is that mach ( mach ) speed of sound varies as temperature. Standard temperature profiles ( absent storms, etc ) will show lower temperatures at higher altitudes.
Lower temperatures result in a lower speed of sound, so a given constant velocity ( airspeed ) results in a mach number which varies as altitude. The higher altitude = lower mach number and vice versa
OR IN SIMPLER TERMS The speed of sound is higher at sea level than at altitude 760 mph at sea level - 660 mph at 35,000 to 60,000 feet. temp is nearly stable 35 to 60 K feet.
would suggest looking at
http://www.fighter-planes.com/jetmach1.htm