I'm seeing this at Gori, after Stavros' briefing for Within Reach. It takes/gives all your equipment and has you staring at the armorer with the cinematic borders on. You can't skip it like other cinematics.
I've had no other issues with the other mission briefings.
Dev Branch version 1.11.115429
In a "realism" aspect, I would agree that the breathing seems louder than "real".
FROM A GAMEPLAY ASPECT the breathing sound is used as a measure of how fatigued your character is. Having it at the current volume lets you hear how fatigued you are or if you've "caught your breath" even when heavy fire is going on around you. It's a replacement for the physical feeling of breathing that is not capable of being passed to gamers (without doing something like camera-bob that can cause negative side effects in players).
IMO, keep it as-is.
I've specifically seen this error with the Ghosthawk and Marshall. I can't give details on the GH, but I was on AW's I&A mission (on their dedicated server) when I experienced the Marshall. I was not the first person in after it had spawned, and do not know the circumstances between it spawning empty and when I found it laying around. Everybody involved was a client, split between different squads or in the same squad, and nobody was a squad leader.
I had just repaired the vehicle, jumped in as gunner, noticed the ammo was full, and moved the turret slightly before jumping out again and switching to driver. A few players then jumped into the gunner slot around the same time. I drove for a bit with 4 "floating" players around me, before getting in range of a target for them to shoot. Despite having "gunners", the turret did not move or play a firing animation while I saw resulting explosions from their combined fire. My clues to the multiple people is that they were all talking to me as if they were the gunner engaging targets or asking me to drive so they could fire.
I got everyone to exit the vehicle, had only one player in the gunner, and still had his body floating/jumping around the vehicle. We encountered AI, and he was shot and killed despite being inside the APC while I stayed alive. I swapped back to the gunner from the menu inside the vehicle and also shot. The ammo used by the other players wasn't reflected in the ammo count I saw on the vehicle.