grammar - "This was the fastest I heard someone [respond/responded]" - which to use, and why?

Here are the two sentences.

1. This was the fastest I heard someone responded.


2. This was the fastest I heard someone respond.

Can someone help me understand:

A) Which one is correct, and what is wrong with the mechanics of the
incorrect one?

B) If they are both correct, can someone tell me what
is the subtle difference between the two different words used?

I chose #1, but my rival chose #2 and everyone agreed with him because they said it "sounds" good. But i'm interested in mechanics and accuracy not sounds. I am really confused with this one. Help from the grammar mechanics gurus is greatly appreciated. Thanks.

UPDATE

Thanks to both Cerberus, Leon Conrad. Cerebus gave the excellent mechanics break-down. The missing That was making it sound awkward.

This was the fastest [that] I heard [that] someone responded.

This was the fastest [that] I heard someone respond.

And Conrad gave the subtle difference between the two meanings.
Based on the context, (from Leon's explanation) I was correct, because I did not hear the response my self, I heard about the response from someone else. Because I cant choose 2 answers, I chose Leon's. That subtle difference was hard for me to spot.

Answer

The second, to me, implies that you heard the fast response yourself.

The first, to me, implies that you heard about the quickness of the response from someone else.

I'd include 'that' before 'someone' in this version for clarity, as @binderbound explained.