Something like a .50 BMG round has a large report and the flight time is just under 2 seconds at a mile, so anything recording audio in the neighborhood would pick up the distinct crack of a rifle prior to the pad explosion. Some of the noise can be suppressed with a big enough suppression device but the sonic boom is unavoidable. Subsonic ammo is simply not workable at these ranges; that's called 'artillery' and it doesn't have sufficient precision to ensure hitting the rocket.
update: yes, I know Elon mentioned the 'quieter bang' sound and it could well be related, but it doesn't sound anything like the crack of a rifle round.
You can listen to the explosion here https://www.youtube.com/watch?v=I6isMuPfxcI
I hear birds chirping. No rifles.
update: yes, I know Elon mentioned the 'quieter bang' sound and it could well be related, but it doesn't sound anything like the crack of a rifle round.