Question

I could not hear the ____ part of his speech​

Answer 1

I could not hear the last part of his speech​ .

Explanation:

Incomplete sentence:

I could not hear the ____ part of his speech​ .

Complete sentence:

I could not hear the last part of his speech​ .

As the situation is clear if the speaker is going away from the listener than than the sound become inaudible and one can not listen properly.

Also if someone speaking not clearly then the speech also becomes inaudible.