The speed of sound is dependant upon air pressure, so it changes by altitude. Most of the time the tip of the stabiliser is higher in the sky than the belly of the aircraft.
The speed of sound is not in fact affected by air pressure - but it is affected significantly by temperature.
The parts of the aircraft that are facing the airflow (nosecone, leading edges of wings & tail) have a huge increase in pressure - but as said this is not the thing that directly affects the speed of sound. What does affect it is the fact that when you compress air that much in a fraction of a second it heats up. A lot. Then there's frictional heating as the air passes over the skin of the aircraft, which at supersonic speeds in not inconsiderable.
The higher the temperature, the faster the speed of sound.