Re: Putting double hungs back in, sound is a factor
Things that will help lessen sound are:
Making sure to bed the glass in putty, so it will never rattle against the wood.
Use a flexible weatherstripping like pile or silicone to cushion the sash against the frame.
1/4" plate glass is probably of minimal value in divided lites where panes are small, up to 12x16, just using double-strength glass would be adequate. If large single-lite, then 1/4" plate would probably have the most effect. Realize that the sash will need to be stronger to carry the double weight of the plate glass.
I've only worked on one house that had 6-over-6 sash with 1/4" plate glass (from 1910), and the sash were 1 3/4" thick.
Remove not the ancient landmark, which your fathers have set.