Question

Correct sentense:1.let us hear his speech

Answer 1

This is BARE INFINITIVE CASE (The infinitive without to). We use an infinitive without to after see or hear + an object. For example: We all heard the bomb go off.

 

So according to the rule the answer should be as follows:

Let us hear him speak.  

 

Answer 2

"Let us hear his speech." Is the correct sentence.
Beside this we can also use
"Let us listen to his speech." Which is also correct sentence.