Think about it this way. Sound propagation in air is a wave. Air molecules at 20 degrees c have a velocity (RMS) of 502 m/s. They are "bouncing around" at this speed. The actual air molecules do not travel with the sound wave. So the sound wave (which is 343 m/s in this temp air) must use this velocity to "bounce" the wave from one molecule to another, on average. The only way to increase the available speed is to increase the RMS velocity of the air molecules by raising the temperature. Adding pressure, and subtracting some heat to keep the temp the same, will raise the amount of medium available to propagate the sound (louder), but will not increase its max speed through the medium because the sound cannot propagate any faster than the air molecules are bouncing around.