Question

change the voice passive or active. we could hear the children wailing outside in the garden​

Answer 1

The voice of the following sentence can be changed into passive voice as shown and explained below:

The children wailing outside in the garden could be heard by us.

Explanation

It was a modal active verb case. When we convert a modal active into passive, we use ‘be’ + Past Participle. The rest of the rules are the same.  

Thing to remember:

An interrogative active will remain interrogative in passive.