In my experience, double glazed sashes are only effective when fitted complete with new corresponding box sashes as well.
I've come across a lot of new double glazed sashes fitted into the old original box frames, and the poor fit due to the irregularities in the old frame, have rendered their noise reduction qualities practically useless. They would definitely need to be fitted with draughtproofing at the very least, as it's these gaps around the parting beads / staff beads / meeting rails etc. through which sound travels.
I think a lot of customers, in London anyway, are conned into having new sashes fitted into the old frames.
In my own opinion, it's far better to keep the original windows and draughtproof them effectively. If noise really is a big issue....if you're on a main road for example, then the only truly effective solution I've come across is secondary double glazing....which is not very pretty in my opinion. Failing that, then new double glazed sashes in new frames.
Personally, I think they look out of place on period buildings.
I work as a domestic carpenter and joiner in and around Greenwich, London.
I hope my comments aren't received too negatively, but I'm often asked by customers (who've spent a lot of money) on new sashes in their old original frames, how I can improve on their noise reduction. I think many of the smaller franchise companies are a racket to be honest with you.