(1) Sound needs medium to spread, and the speed of sound propagation is different in different media. Generally speaking, sound travels fastest in solids and slowest in gases.

(2) When playing the flute, the air column vibrates to make a sound. When the length of the air column is controlled differently, the vibration speed of the air column is different, so it will emit different tones;

(3)∫v = ST，

∴ sound propagation time t = SV = 680m340m/s = 2s;

So, the answer is: different; Tone; 2;