The speed of sound in a gas (assuming ideal gas principals hold) is predicted by this equation:
Speed of sound = (g R T / M)^0.5
g is the Adiabatic Constant, for air ~1.4, for most propellants ~1.23 For helium ~1.66
R is the Universal Gas Constant 8.314 J/mol K
T is the Absolute Temperature (Kelvin)
M the molecular mass of the gas in kg/mol. In the case of air or propellant gases we use the average since the gas is made up of a bunch if different gases. For air it is estimated at .02895 kg/mol. Helium is only .004 kg/mol. I could not find a good estimate for the byproducts of gun powder combustion but it has to be slightly higher than air.
Temperature comes from both the heat generated by the combustion and by the pressure created by that combustion.
As you can see due to helium's higher Adiabatic Constant and low molecular mass it has a very high speed of sound even at room temperature but remember in a light gas gun the first stage compresses the helium very fast and you get significant heating of the helium from adiabatic compression (adiabatic here simply means we are assuming no significant heat lost or gain to the environment) This is a safe assumption give how fast these event happens.
-rambling
ETA: I am a solid-mechanics/dynamics focused mechanical engineer and this is rapidly reaching the limit of my knowledge of the dark side (fluid/thermal) of mechanical engineering. It is dangerous to learn too much about the dark side...