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It's not a complete vacuum (hard vacuum) inside the vacuum cleaner, just a lower pressure.
The loudest sound possible in a medium is when the pressure waves alternative between complete vacuum and 2x the pressure of the medium. In standard atmospheric pressure it's about 194dB. Inside a vacuum cleaner it would be slightly less, but you'll never achieve sound that loud, so it doesn't really matter.
TL:DR: No difference in sound levels inside a vacuum cleaner.
It's not vacuum inside the VC. It's full of air and dust, so it's just as loud inside as it's on the outside.
It's not a total vacuum in there, despite the name.
There's a (very loud) motor that pumps air out of the machine at an incredible rate, and air rushes in to fill that space, mostly through the only available opening (the hose). The rushing air carries debris with it.
So there's always some air inside the canister (or whatever collection container), which allows sound to propagate through. My guess is that it would be deafening in there.
A big fan moves air out of the cleaner giving it a lower pressure inside but not a total vacuum, because air comes in from the handle side (the thing that sucks) to replace it. A bit of air leaks in from other parts of the vacuum too.
Since there's still some air in there it's not like there's no sound.
