Quoting Tdscanuck (Reply 18): Speed of sound in most gases is a function of the temperature (and the gas) only. Assuming we're talking standard air at normal pressures all the time, then it's just a function of temperature.

Quoting Wardialer (Reply 2): How can this be done using a calculator or something....???? 
Mach is just a unitless number that is a ratio of relative speed to the speed of sound (at that altitude). As stated before, it's a function of temperature, and temperatures change with altitude.
The equation for Mach number is
M= u/a,
where u is the velocity of the aircraft,
a is the speed of sound. a = (ɣRT)^(1/2) (square root of (ɣRT)).
ɣ = 1.4 (for air)
R = 287.058 J/KgK (for air, and metric) or R = 1716.6 ft*lbf/(slug*R) (for English)
T = temperature, either in K, or R depending on if you're using metric or English
Here is a standard atmospheric table.
http://www.pdas.com/e2.htm
It's the only one I could find in table form, but the temperature is in Rankine, so you need to keep that in R if you're using English, or convert it to K if you want to use Metric. So you can now calculate Mach numbers for any altitude if you'd like, but you need to have the flight velocity. Just choose one, or look up typical cruising speed.
You can always rearrange the equation and calculate the velocity of the a/c if you know the Mach number, and what altitude it's flying at. Anyways, I know this is way more in depth than you were looking for, but I hope this helps. It's really not that hard. If this doesn't make sens I'll try to clarify tomorrow.