The speed of sound in an ideal gas depends only on its temperature and composition. The speed has a weak dependence on frequency and pressure in ordinary air, deviating slightly from ideal behavior. In colloquial speech, speed of sound refers to the speed of sound waves in air.
The speed varies depending on atmospheric conditions; the most important factor is the temperature. Humidity has little effect on the speed of sound, nor does air pressure by itself. Air pressure has no effect at all in an ideal gas approximation.
