(1) The sound of the piano is produced by the vibration of the strings; Sound needs media to spread, and the sound of piano reaches people's ears through the air.

(2) Sound can propagate in gas, liquid and solid, but the propagation speed is different. Generally speaking, the propagation speed is higher in solid, followed by in liquid and lower in gas; Sound cannot travel in a vacuum.

So the answer is: vibration; Medium; Air; Solid; Vacuum.