The moment I noticed this, I decided to try and fix it myself, the new pyro voice lines sound nothing like the orginals, however I mimicked the same muffled effect from the old lines and here!
Enjoy, just put the vo folder in sound, inside of your custom folder, no vpk for now.
I wonder if I could submit this to valve so they can fix the official lines...
This is why I'm thankful for people like fixing fixing simple things Valve's laziness has caused. May you be blessed with many hats for this god-tier mod, for it is an essential mod to anyone with TF2!!!