No, he will not be able to hear the sound produced by his friend.

When object vibrate sound is produced, these vibrations cause the nearby molecules to vibrate. These vibrations are nothing but to and fro movement of particles. When these vibrations reach our ear we hear the sound.

When an object travels at a speed higher than that of sound in air, it produces shock waves. These shock waves carry a large amount of energy. The air pressure variations associated with this type of shock waves produce a very sharp and loud sound called sonic bloom or supersonic speed.

The number of vibrations (complete waves or cycles) produced in one second is called frequency. Its SI unit is hertz.

No, sound cannot travel through vacuum.
Because, sound can travel only when there are particles, which can be compressed and rarefied in the medium.

An object should- vibrate to produce sound. The vibrating body causes the medium around it to vibrate.

Reason: In solid, particles are packed closer than in liquid and gaseous. So sound waves travel easily in solid. Sound travel faster in iron.

Tuning fork

Velocity of sound = 340 ms^-1
Height of tower = 750 m
Total distance travelled = 2 x 750: 1500 m
\(Valocity=\frac {distance}{time}\) 
\(340=\frac {1500}{time}\) 
\(time=\frac {1500}{340}=4.4s\)
After 4.4 s, the splash is heard.

The frequency of 600Hz means that the source of sound vibrates 600 times in 1 second.
1 minute = 60 sec.
Number of vibrations in one second = 600
∴ Number of vibrations in 60 seconds = 60 x 600
= 36000
Hence the source of sound vibrates 36000 times in a minute.