Are some ammos louder than others? More importantly, are there some that are more quiet than others? I have not done any solid tests but it seems to me that I have shot some brands that are much louder (and have a larger flash) than others. So, what gives? Is it possible?

Yes.

There are many variances within .223 ammo. Some of it is loaded to military specs, such as Federal XM193 or Winchester Q3131A. This ammo is loaded hotter than commercial spec .223 and in many cases will lob a bullet of the same weight 200 fps faster at the muzzle. Being that it is more powerful, it is also louder and has more kick. Some of the military loads contain flash inhibitor, some do not. And some contain more than others. The surplus South African military stuff seems to have the least flash of any military spec .223, with the Q3131A next and the XM193 with the most. But that extra noise and power also propels the bullet at a significantly higher velocity, making it a much more effective terminal performer. As far as I know, there is no .223 round (at least available to civilians) that is what you could call 'quiet'.

-Charging Handle

It's all powder and weather related.  Sticks, flakes, spheres all perform differently.  Temperature and humidity affect how the powder burns and the sound you preceive.

"...more quiet than others..."

Try "quieter than others"

I wouldn't know a Db if it hit me.

Muzzle blast is loud--- period---. Buy a suppressor, then you will have taken care of that problem, but I do not think MN is a suppressor state, is it?