It's referring to a voice which has been mentioned previously.

English Teacher
Hi folks,
Could anyone explain to me why the definite article is used for the first ‘voice’? It’s from A Minute to Midnight by David Baldacci.
They got out and looked around.
“See anyone?” asked Blum.
“No. No cars, either, but the garage might be around back.”
“Can I help you?” said the voice.
They turned to look around for the person.
“Can I help you?” said the voice again.
Thanks as always,
emp0608
It's referring to a voice which has been mentioned previously.
I am not a teacher.
It hasn't been mentioned previously. That's why I find the use of the definite article very unusual.
Here's a longer quote:
The home was large, new, and contemporary in design, with lots of metal and glass.
“Looks like something you’d see a tech mogul build in Palo Alto,” commented Pine as she slowed the SUV and pulled to a stop in front of the house.
They got out and looked around.
“See anyone?” asked Blum.
“No. No cars, either, but the garage might be around back.”
“Can I help you?” said the voice.
They turned to look around for the person.
“Can I help you?” said the voice again.
The sound seemed to be coming from the front door.
“They must have one of those video security systems,” said Pine. They stepped up to the front door and Pine held up her badge.
We use it for speech coming from a machine.
When they first heard the voice, they did not know the origin. I think the definite article is inappropriate there.
Typoman - writer of rongs
Yes, it is odd. The writer apparently wants to make reference to the voice as if the reader already knows the identity of the voice. It's hard to guess why he'd want to do that, but that is what he wanted to do. Perhaps later context might shed some light on that decision.
They use it for person too, so I guess they are thinking of it as unique.
I believe "around/round back" is OK in AmE.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.