Zimmerman's father, Robert Zimmerman, took the stand during a bail hearing for his son.
Mark O'Mara, George Zimmerman's lawyer, played a tape of a 911 call in which a woman tells the 911 dispatcher that someone is yelling and she thinks they are calling for help.
At least 14 calls for help within a 40-second period can be heard in the background of the call. Suddenly the cries for help stop and the woman tells the 911 dispatcher that she just heard a shot.
Zimmerman, 28, is charged with second degree murder in the shooting death of unarmed teenager Trayvon Martin.
Martin's parents insist that the voice calling for help belongs to their son.
Both Zimmerman and his father appeared in court with the distinct outline of bullet proof vests beneath their suits.
Earlier in the hearing, a member of an ambulance crew told the court that Zimmerman's head, including his moustache and beard, were covered in blood after the shooting of Martin. EMT Kevin O'Rourke testified that 45 percent of Zimmerman's head was covered in blood and that the lacerations on his head would likely need stitches.
Zimmerman maintains that he shot the unarmed teen after being knocked down, having his head slammed on the pavement and then believing that Martin was going for Zimmerman's handgun.
Zimmerman will have to stay in jail while he waits for the judge to decide whether he will be granted bond.
Circuit Judge Kenneth Lester revoked Zimmerman's $150,000 bond earlier this month when prosecutors told the judge Zimmerman and his wife misled the court about how much money they had during the April bond hearing.
Prosecutors said a website Zimmerman created for his legal defense had raised $135,000 at the time of his first bond hearing. Zimmerman and his wife did not mention the money then.
ABC News and the Associated Press contributed to this report.
