Can Defense Make Oscar Pistorius Believable?

Ballistics, audio experts slated to take the stand in Pistorius trial.

May 2, 2014— -- Oscar Pistorius got pummeled in court last month, brought to tears repeatedly by prosecutor Gerrie Nel’s attacks.

But with his murder trial resuming Monday in South Africa, sources close to the defense tell ABC News that the heart of their case hasn’t begun. As many as 11 additional witnesses are expected to be called during two weeks of testimony. A ballistics expert will be among those witnesses to back up Pistorius’ version of how he shot his girlfriend, model and reality TV star Reeva Steenkamp, on Valentine’s Day 2013.

Pistorius claims he mistook his lover for an intruder and shot her through a locked bathroom door.

A forensic audio expert is also likely to try to convince the judge that earwitnesses - neighbors who heard screaming and sounds they thought were gunshots - may have mistaken gunshots for Pistorius hammering the locked bathroom door with a cricket bat to get to his girlfriend after he shot her. Those witnesses may also have mistaken Pistorius’ screams after realizing what he had done for a woman’s screams of fear, the audio expert will be expected to tell the court.

Sources tell ABC News that Pistorius, who has pleaded not guilty, has spent the past three weeks recuperating and resting from Nel's verbal mauling. The prosecutor, known as the “Bull Terrier” for his aggressive questioning, was able to trip up Pistorius, the paralympian sprinter known as the Blade Runner.

Pistorius, 27, seemed to stumble when discussing the night he killed his girlfriend. Nel was on the attack, telling the athlete, “Your version is so improbable that it cannot be reasonably, possibly true.”

Pistorius seemed to change his testimony when grilled by Nel, saying his gun just went off that night.

“I didn’t shoot at anyone,” he told the court.

He previously testified that he was firing at an unknown intruder hiding in his bathroom.

Pistorius faces 25 years to life in prison if convicted of premeditated murder.

Pistorius' fate will be decided by Judge Thokozile Masipa, who will be assisted by two assessors.