Speed of Sound in Different Media

Which sound travels faster-the roaring of a lion or the buzzing of a mosquito? Justify your answer.

In which of the following materials is the speed of sound maximum?

Recently, ISRO had tested its Scramjet engine with the help of Advanced Technology Vehicle, named as RH $560$ which was able to fly at MACH $6$. What do you understand by this statement? Also calculate its speed in $\mathrm{km}/\mathrm{h}$.