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The speed of sound in an ideal gas is given by the equation:
R - the universal gas constant, 8.314 J/(mol K)
T - the absolute temperature, K
M - the molecular mass of gas, kg/mol
gamma - the adiabatic constant of gas.
You can find the calculator below.
Its default values (adiabatic constant 1.4 and molecular mass 28.95) are given for dry air.
For a monoatomic ideal gas, the adiabatic constant is 5/3, and for a diatomic gas (such as nitrogen and oxygen) adiabatic constant is 7/5. For molecules with three atoms, it is 4/3.
With an adiabatic constant 5/3 and molecular mass 4 g/mol for helium, the sound speed at 20C is 1007 m/s. That's why the voice in helium sounds so funny.