Note that that's extra tricky, because there are two generally sounds: the first received will be from the shock wave of supersonic bullet, then the report from the rifle.

Only if its a supersonic projectile AND its still supersonic when it passes the mic. Assuming a 115gr 9mm +P (.167 BC) that leaves the muzzle at 1275fps, the bullet slows to subsonic velocities between 50 and 75 yards. Assuming standard day at sea level.

Likely they’re looking for the muzzle blast. Also, rifles generally aren’t used to commit crimes, so looking for the shockwave exclusively will be next to pointless.