It's only going to be silent in a vacuum. The stated 8,000 fps is about 9 times the speed of sound. I suspect that much energy turns into a real loud crack.
I forget to mention that the "no recoil" claim is also going to be bogus unless they've found away around Newton's laws. Even "light" recoil adds up if fired fast enough. 5.56x45 is not generally considered a heavy recoiling round and compared to the weight of a mini-gun should be negligible. While I was in the Army I talked to some guns with the 9th Infantry Division air defense battalion. As an experiment for the "Light Division" role they tested a mini-gun on a pedestal mount for a jeep. Turned out that sustained fire to either side of the vehicle would tip the jeep onto it's side.I classify that information as a "B" source. I suspect that they actually stopped firing before completely tipping it but a noticeable rock is very likely. At that's at a typical rate of fire of 1,000 rpm.
Hey it's the Army....I bet they tipped er over Remember Puff the Magic Dragon from the Vietnam era? It was tough to aim effectively as it sent the airplane sideways (from recoil) and up (from loosing weight so fast). Of course when you are putting that much ammo out I guess accuracy comes second (unless you are the one in front requesting close air support )Pat