Apparently, the npcs from half life 2 that you can spawn in portal 1 (I.e. npc_zombie, npc_citizen, etc no longer have working voice files. I'm pretty sure it was not always like this, as a few years ago, when I got portal 1, they had voices/sounds.

On a similar note, any npc holding a gun cannot hurt me when shooting it (which is also silent), it just shoots right at me and i take no damage whatsoever.... but a charged whack from a stunstick wielding metrocop or a gun bash does (also silent still).
If anyone can help, that would be amazing. I have recently ported my mod over to using portal 1's engine for ease of having working npcs, but if this is going to happen, I don't know what I'll do...

Solved the sound issue, except for the npc's guns. Metrocops and combine soldiers still don't have sounds for shooting guns and don't damage me.

Did you turn off the regen? I think that also disables damage under a certain threshold.

I don't really know how to turn off the health regen, but I don't think that would be the issue since two npcs with pistols can have a silent fight, not shooting any bullets (just a muzzleflash and animations), but if you give them both stunsticks, they will whack at eachother until someone dies.

Hm. You might try copying the HL2 weapon scripts over the Portal ones, because something might have been changed there accidentally.

Just tried that, no luck. could it be an issue with the game binaries?