George Zimmerman Update: Audio expert testifies accused killer Zimmerman is likely not the person screaming in 911 call

(CBS/AP) SANFORD, Fla. - An expert in voice identification hired by an Orlando newspaper testified Friday that screams for help heard on a 911 call made the night teenager Trayvon Martin was killed do not match the voice of his accused murderer, neighborhood watch volunteer George Zimmerman.

PICTURES: George Zimmerman in court

READ: Trayvon Martin Shooting: A timeline of events

Tom Owen testified on the second day of a hearing that will determine whether voice identification experts can be used at Zimmerman's second-degree murder trial for fatally shooting Martin in a gated community in Sanford, Fla., on Feb 26, 2012.  The trial starts Monday and Zimmerman is pleading not guilty, claiming self-defense.

Owen was hired by the Orlando Sentinel last year to compare a voice sample of Zimmerman with screams for help captured on 911 calls made by neighbors the night Martin was killed. He said Zimmerman's voice doesn't match the screams. He only compared Zimmerman's voice to the 911 calls because he didn't have a voice sample for Martin at the time.

"The screams don't match at all," Owen said. "That's what tells me the screams aren't George Zimmerman."

Owen said that in general, it was much easier to make a positive elimination than an identification. On cross-examination, however, he said that the amount of screaming was enough to definitively rule out George Zimmerman.

"I didn't make a positive identification or elimination in this case," Owen said. "I made a probable elimination it was not George Zimmerman making the screams."

Answering a line of questioning from the defense, Owen also said that he makes a commission on the sale of the software which he used to analyze the call.

Owen also testified that remarks Zimmerman made in a conversation with a police dispatcher aren't a racial slur. He testified Zimmerman said, "These f------ punks."

An FBI expert testified Thursday that there wasn't enough clear sound on the 911 recordings to determine whose voice it was. Hirotaka Nakasone said that the screaming lasted for about 18 seconds - however, the amount of screaming that was not "stepped on" by the voice of the 911 dispatcher or caller was only about three seconds.

He said because the screaming voice was affected by distress or emotion, there was no way to compare it to "reasonable, natural speech and come up with correct answers."

"The screaming was not normal - it was pretty much uttered by someone who was facing probably imminent threat of death," he said. "Very difficult to analyze."

The screams captured on the 911 calls are crucial pieces of evidence since they could determine who the aggressor was in the confrontation. Martin's family contends it was the teen screaming, while Zimmerman's father has said it was his son.

Experts have come to mixed conclusions about the screaming in the background of the 911 call. One state expert, who could prove crucial to prosecutors if the judge allows the jury to hear his testimony, said he heard Martin screaming and saying "I'm begging you" in the background of the call.

Complete coverage of the George Zimmerman-Trayvon Martin case on Crimesider