You ask how is this revelent to the investigation? Last week Michael Brown's autopsy was released and it revealed that he was shot 6 times. According to the autopsy report the fatal shot was a single shot to the head, the last of the six shots to his body.
This evidence along with other video/audio recordings and witness statements match with the officer's statement will prove the credibility of his statements and hopefully answer questions about his final moments.
Take a listen to the alleged audio below: