David, I have a 400 hp diesel electric generator in one room of my shop that supplies power to my 480 VAC equipment. As you can imagine, it is quite loud (as in have to shout into someone's ear to be heard when it's running and you are standing next to it)

I used a multi-step approach on sound insulation that was quite effective.

Step 1 was open cell spray foam in the walls and ceiling.
Step 2 was to staple some sill-seal foam to the studs so as to isolate the drywall boards from the studs and minimize harmonic transfer.
Step 3 was installation of two layers of drywall.
Step 4 was installation of the sound deadening board on the outside of the drywall.

For the inside of the doors, I applied the same treatment as the walls.

I also isolated the generator from the slab with a layer of 1/2" felt and two different densities of foam.

It's been quite effective. 99% of the noise is contained in the room.

Re the baffles in the vents, this approach can be very effective. It is what generator companies use to help trap the noise that emanates from generators used in quiet environments (hospitals, movie sets, etc).