In one of the activites you treated 'sound' as contable noun. But according to your experts 'sound' is a uncountable noun. Plese clarify.

This concept might confuse you a bit but with passage of time and more practice in English you would surely get a strong hold in this.

Sound is a word which can be treated as a countable as well as an uncountable noun. 

Here are examples for your better understanding:

Countable:

- I could not sleep because of the various sounds coming from the jungle.

- The sounds of the different instruments kept ringing in my ears till late.

Uncountable:

- Sound travels slower than light. 

- Decible is a unit to measure sound. 

- Sound can travel long distances.