"Oh my God! We've got to hurry!" Tanya Brown said on a recording of the call she made as she raced to a nearby medical center with her nephew, Iran, who survived the shooting.
The boy's mother sobbed heavily as the tape was played and fled the courtroom.
Muhammad, who is acting as his own attorney, sat with his chin in his hand and did not ask Tanya Brown questions.
Iran Brown and Caroline Seawell, another sniper victim who survived, testified Tuesday as Muhammad's trial for six Maryland murders continued. Already on death row in Virginia for a sniper killing there, he faces possible life in prison if convicted.
Brown described the pain he felt when the bullet destroyed most of his stomach and some of his spleen.
"I couldn't breathe and I was scared," said Brown, now a 17-year-old high school junior.
Seawell said she was loading Halloween decorations into her minivan when she was shot Oct. 4, 2002, in the parking lot of a Fredericksburg crafts store.
"I dropped to the ground and prayed that God would let me live so that I could take care of my kids," she said.
Prosecutors are building a chronological case against Muhammad, detailing the 10 sniper murders and three woundings that began Oct. 2 and ended with Muhammad and accomplice Lee Boyd Malvo's arrest three weeks later.
Muhammad is charged with six Maryland murders, but prosecutors are detailing other shootings to establish a pattern.
Maryland prosecutors bill the new trial as insurance in case Muhammad's Virginia conviction is overturned. Accomplice Lee Boyd Malvo's trial is set for the fall; Malvo is serving a life term in Virginia.