I have one of the Ridgid mufflers, and it helps a little. I always
wear hearing protection. I've found that strapping a piece of open
cell foam over the top of the motor quiets the whine a little, and
keeps the exhaust fan from blowing more dust into my face. It's thin
enough and porous enough that it doesn't overheat the motor, even when
run for extended time periods.
I've heard of some people building a box for their shopvac out of thin
plywood or hardboard, or even pegboard for more ventilation.
Supposedly this helps a lot with noise. I'd guess that enclosing the
shopvac in a cardboard box would help, if it was exactly the right size
and the wheels could stick through the bottom. Probably wouldn't last
very long, though.