Is there any packing material in the stock mufflers?
If there is fiberglass or equivalent, it could get matted down over time and the mufflers would indeed get a little louder.
And of course, parts do wear out, parts of the baffle could crack or open up a bit.
I haven't noticed either, but this sort of thing is too subjective. Not to mention when something happens so gradually the person most exposed to it would be least likely to notice right?
