SANFORD, Fla. -

At the heart of Monday afternoon's scheduled 4 p.m. audio-expert hearing is a 911 call placed the night George Zimmerman shot Trayvon Martin. 

Zimmerman, a neighborhood watch coordinator, claims he killed the unarmed teen in self-defense following a scuffle. 

SPECIAL SECTION: George Zimmerman trial

On the 911 call, you can hear someone screaming in the background before the fatal gunshot.

Audio experts have been divided on who that was. The FBI even said they could not determine if it was Zimmerman or Martin.

But the state has already presented an expert who produced a controversial report saying not only can he definitively say the voice crying for help is that of Martin, but that he can hear the late teen saying the words, "I'm begging you."

You can understand why the state wants a jury to hear from that expert and why the defense does not. 

Especially when you listen to potential jurors like H13. Here H13, a white woman in her 60s and grandmother of four, is conversing with defense attorney Don West.

West: What is your sense of what happened after that?

H13:  Honestly, I don't know what happened after that. I know there was the 911 call and I don't know what transpired.

The difficult reality of this case is that no one was witness to that deadly moment in time. That's what makes the 911 call so crucial, and why Monday will be the fourth day the hearing over audio experts will be discussed.  

The question at the hearing centers on the science behind the experts' conclusions: was their methodology 'new and novel' or generally accepted by the scientific community? 

That's the basis for Judge Debra Nelson's highly-anticipated ruling.

The hearing is scheduled to begin at 4 p.m. with a California-based defense expert. Defense attorney Mark O'Mara believes it could take three to four hours.