The most effective sound reduction approach is triple pane glass windows and drywall ceilings on resilient channels with as few openings as possible. I doubt you would be able to detect the difference between fiberglass and rockwool attic insulation.

Don't know what window style you have, but in general, casements seal more tightly with better sound insulation than do double hung. Look for quality windows and expert installers.

It's hard to find real numbers for your issue. I just removed old mineral fiber insulation from the attic of a renovation I am doing. I was surprised how heavy it was. I could hardly lift a full trash bag of it. Anyway, here's one source I found. It does seem there's a real benefit for sound reduction.

Blown in cellulose will outperform the fiberglass for the same cost or less.

If you have the original windows, restore them and don't ruin the appearance and value of the house with replacements. If you chose to take out original windows - save them! Put them in storage so the next owner can restore the house to it's original beauty.