The issue is most prevalent with L4D1 characters, for example Louis' "Taki" or Francis' "Nerd" lines used to be useable for me but after getting back into the game pressing them no longer makes any line be spoken. I tested this with a friend in a local server, the lines still cannot be heard by me.

Strangely, my friend can hear them on his end, so in essence I think the "missing" voicelines are still playing for other players, but I can't hear them or the responses from other characters for some reason. I tried disabling all mods except for Vocalizer, and it still doesn't work.