Understand velocityor speed of a sound wave?

Does it always stay the same or around the same value (speed of sound), because i was thinking extremely shortly after a disturbance was made the wave would have to pick up speed i am thinking as if the sound wave goes from 0 to 340(or something )ms then when the wave is about to stop it slows down or does it start instantaneously at 340 and stop at 0 if so how ??? this whole speed of sound is confusing me so bad