What is a heartbeat? At its most basic definition, it's one complete pulsation of our blood-pumping organ.

We all know a heartbeat is so much more than that.

"It means life, doesn't it?" said Linda Ahern.

In the months Micah Ahern courageously fought cancer at Cook Children's Hospital, his mother Linda often slept right by his side. The rhythm of his heart filled hers night after night.

"I think to a lot of people, heartbeats sound the same. But it doesn't to me. His heartbeat sounds different to me. It feels different to me," she said. "So much sweeter."

As hard as Micah fought, the cancer proved too much.

"We were just sitting together on a beach talking, and he was telling me, you know, 'I'm really tired. And I'm worn out,'" Ahern recalled.

In the weeks leading up to Micah's death, the family tried to make as many memories as possible. There was one memory Ahern didn't realize she could keep after Micah passed away.

"To be offered the gift to hear his heartbeat after he was gone—I mean, you really cannot quantify that," she said.

The child life specialists at Cook Children's consider it one of the most meaningful things they can do for a family that's about to lose a child: to record their child's heartbeat and turn it into a song the family can keep forever.

"It's such a personal and private memento for them," said Shea Ingram, a music therapist at Cook Children's.

Ingram brought the idea to Cook a few years ago. She uses a powerful stethoscope to record the heartbeat. Then, along with music producer Raymond Turner, they work with the family, and sometimes, even the child, to choose a meaningful song or tune. Without altering the heartbeat in any way, they then set it to the music.

"I'll tell you, each one impacts me in a very strong way," Turner said. "There have been times where I'll listen to it, and I'll have to kind of stop for a minute and just let the tears come."

Turner, who's responsible for putting the songs together, knows the parents' pain firsthand. He too has lost a child.

"It's very special to me, so I take very particular care in handling that, knowing it's something that'll be with the parents forever," he said.

"It's a sacred moment. The whole process is for the families," Ingram said.

Not every family who's offered this wants it. Sometimes it's just too tough, and that's okay. For the 10 families so far who have wanted it, it has meant the world.

Micah Ahern chose his own heartbeat song before he died. Linda doesn't listen to it often. It's still hard, but knowing it's there when she needs it is enough.

"For a mother who expects and plans to go before her children, it is really a gift you can't even explain," she said.

On this day, she shared his heartbeat song with WFAA.

"For me, it's a reminder that he's not here, but he's alive," Ahern said. "It's awesome. It's really like the sweetest sound. Isn't it?"

It's the sound of a life and the lives it continues to touch.