Speed of sound mainly varies with temperature differences. Not air pressure.
Wikipedia:
Sound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how much distance such a wave travels in a given amount of time. In dry air, at a temperature of 21 °C (70 °F) the speed of sound is 344 m/s (1238 km/h, or 769 mph, or 1128 ft/s).
Although the term is commonly used to refer specifically to air, the speed of sound can be measured in virtually any material. The speed of sound in liquids and solids is much higher than that in air.
In the Earth's atmosphere, the speed varies with atmospheric conditions; the most important factor is the temperature. Since temperature and sound speed normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source.[1] Air pressure has almost no effect on sound speed. It has no effect at all in an ideal gas approximation, because pressure and density both contribute to sound velocity equally, and in an ideal gas the two effects cancel out, leaving only the effect of temperature. Sound usually travels more slowly with greater altitude, due to reduced temperature, creating a negative sound speed gradient. In the stratosphere, the speed of sound increases with height due to heating within the ozone layer, producing a positive sound speed gradient.
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