George Zimmerman Trial: Former neighborhood watch captain back in court Friday as audio testimony continues

(CBS/AP) SANFORD, Fla. - For a second day in a row, a Florida judge is weighing whether to allow certain voice experts to testify at George Zimmerman's trial.

PICTURES: George Zimmerman in court

READ: Trayvon Martin Shooting: A timeline of events

Zimmerman is charged with second-degree murder for fatally shooting 17-year-old Trayvon Martin during a struggle in a gated community where he lived last year. He is pleading not guilty, claiming self-defense. Zimmerman appeared in court Friday for the hearing.

Circuit Judge Debra Nelson is listening Friday to testimony from voice experts to determine whether witnesses with expertise in speech identification should be allowed to testify when Zimmerman's trial starts next week.

Neighbors called 911 during the fight and cries for help can be heard on the recordings, which will be important pieces of evidence. One state expert claims he heard Trayvon Martin screaming and begging in the background of the call. This is evidence that could prove key for the prosecution if the judge allows jurors to hear it.

Martin's family claim the cries came from the teen, while Zimmerman's father has testified they were those of his son. Zimmerman's brother told CBS News' Crimesider that the state expert's findings amounted to "voodoo forensics."

First to take the stand Friday was Tom Owen, a New Jersey voice expert who analyzed the 911 tape for the Orlando Sentinel and ruled out George Zimmerman as the source of the screams after comparing it to Zimmerman's speech.

On Thursday, an FBI voice-recognition scientist said that he didn't believe it was possible to identify who was screaming in the background of a 911 call because of the tape's quality.

Complete coverage of the George Zimmerman trial on Crimesider