Even with a foam muffler on my Shop Vac, (‘Vac available from Amazon) it was still louder than I wanted without hearing protection. I tried putting a piece of acrylic over the front of it, but when I had a few pieces of scrap wood available I decided to box in two of the sides as shown in the photos below. I attached scrap packaging material to the inside of them to act as noise-insulation.
I could probably box it in even more, but at this point I’m getting a noise level of under 80 decibels where I generally stand. Because it’s hung from my FastTrack shelving unit in two parts, I can easily lift the front or side to turn the vac on or fix any problems (see my Fasttrack setup howto here).
I wouldn’t call it “whisper quiet” by any means, but probably good enough that it’s not an “active project” anymore. If I’m running a really long machining cycle on my CNC router I’ll still use hearing protection sometimes, but mostly it’s tolerable.
For the most part, my Shop Vac noise insulation setup is done with scrap material. I’m always in favor of environmentalism that saves me money (and my hearing in this case). Maybe there’s a market for a commercial product like this. On the other hand, if you need your ‘Vac to be mobile, a box like this guy built might be what you need. He seems to have used scrap as well, although it’s carpet in his case.