Actually i instigated that Vulcan into firing a few hundred rounds by depressing the firing switch.
Gotta love crosstraining.
However you have to be downrange of the weapon a bit to hear the sonic report over the muzzle blast of the propellant charge. I am happy to report that i did not stray downrange the day we were firing the Vulcan PIVADS, lol.
Any projectile that cracks Mach 1 will have a supersonic report(which is why it's fairly pointless to suppress most rifles), but the larger the projectile/faster the speed, the louder the report will be.
Normally a supersonic projectile makes a tearing sound as it passes overhead/by. I'm thinking that it's the massive ROF of mini-guns and rotary cannons that makes it sound more like a growl than a tear.
A large scale- Battalion or more- FFE of Artillery passing directly overhead makes a similar sound(VERY LOUD in person).
Mudd did some FAC-G work in his youth, i'm sure he's familiar with the sound i'm talking about.
It's way cool.
EDITED TO ADD: I Just listened to the clip. Definitely the sonic boom of the rounds that's making the growl. The initial report is the propellant charges of the cartridges, the brrrrrrrrrr afterwards is the sonic report of 70rds per second tearing ass downrange. A large arty barrage does not have the same frequency of growl, it's more like a slow motion version, and is a lot higher pitched.
"It should come as a surprise only to the fools among the men of our generation that we liked war."
Major V. 'Popski' Poniakov, British LRG, WWII