Hi,

I think you are answering your on questions.

A muffler is a pipe with more pipes inside that let exhaust get redirected into each other’s paths by hitting baffling. A rattle can occur when those internal supports don’t support. (:)
On rare occasions it happens before the outside shows rust or even a hole.
If you have some idea hoe the muffler is constructed you can try stabbing or stabilizing a baffle or pipe with a screw for a while in a pinch.
Eventually it will get worse especially where the screws penetrate the housing.

A answer could be … If the engine spins faster it should pump out more air and there should be an increase in volume or flow from that alone and not so much noise.

Which one are you questioning?
A collapsed muffler could be quieter, to some extent, but that usually means it’s more restricted too.
The flow rate is not so much a change because the faster gases coming out will be hotter and therefore thinner or not as dense.
The gases can be taking a shorter route through the muffler to be getting out faster too or be building up back pressures.

Overall, it’s a tough call but if you can shake the muffler and it rattles then it’s not as designed or desirable. Replacement is on the horizon.

Phil