The FBI has reportedly interviewed a man who lives nearby where Brown was killed, who says he was speaking to a friend using a video chat service at the time of the shooting.
In the recording, a series of six gunshots can be heard, followed by a pause, then another four quick shots, said forensic audio expert Paul Ginsberg.
The man, who asked to remain anonymous, came forward through his lawyer with the recording.
"I was very concerned about that pause... because it's not just the number of gunshots, it's how they're fired," the man's attorney, Lopa Blumenthal, told CNN. "And that has a huge relevance on how this case might finally end up."
The audio recording, if authentic, appears to be the first of the incident itself. Ferguson police have reportedly ordered dash and body cameras, but they have not yet been deployed, and no video of the shooting is known to exist.
Supporters of Brown say he was confronted by the officer, who chased him down the street and shot him while his hands were in the air, while Wilson says he was firing in self-defense as Brown "bum-rushed" him.
The pause in gunfire may corroborate witnesses' story that Wilson fired deliberately at an unarmed Brown. But it could also back up Wilson's account that he only shot to kill Brown after the teen charged at him.
Autopsies show Brown was hit by at least six bullets. Four shots to his arm and side were survivable, but the last, which struck him at the top of his head, was likely immediately fatal.