Back in my competitive shooting days I enjoyed shooting in High Power Service Rifle matches. I shot a Garand along with a couple of other guys, but most of the line used Springfield Armory M-1A rifles (civilian version of the M-14). We did have a couple of National Match M-14 rifles (semi-auto only, selector slot welded and trigger sear exchanged) as well. Part of our duties as competitors was working the pits for the other relays. Our range was 300 yards, since getting a full 600 yards was a bit cost prohibitive. Still, when you're working the pits on a 40 position line with a shooter at every position you had better be wearing hearing protectors. During rapid fire you have 40 shooters firing 20 rounds each (800 rounds) in a period of 2 minutes. Not necessarily MG level, but enough to cause serious hearing and stability issues for the unprotected.
The sound of the bullets passing overhead was a very clear, very sharp, very loud "CRACK". You could also hear the faint "thump" sound of the rifles, and a slightly louder "whump" of the bullet hitting the backstop. If you heard the bullet hit paper (difficult due to the bullet crack, but possible) it was a slight "pop" sound. The only real way to tell if the bullet hit the target was to see the tiny pieces of paper floating down.
Very few of the firearms represented in RO would be subsonic in any of the maps where infantry is used. None of the tank guns (except the short 75mm on the German Pz.IV D) would go subsonic on even the longest ranged tank map. Basically everything should make a sharp "CRACK" as it passed by, followed by a muffled "boom" at extended ranges. The only problem I can see is that to make them sound realistic they need to be LOUD, as in painfully loud.
Gunshots and supersonic bullets are LOUD. Louder than is pleasant, in fact loud enough to do permanent damage to your hearing. You'll have to take my word on this, but I do have personal experience - four shots from my Garand while not wearing hearing protection left me with permanent hearing loss in my left ear.
