Adding something for zombies to say could be a lot of flavor for relatively little work. And it would be easy to contribute more - just json text.
The only problems from the code side would be making zombies not aggro at each others' sounds (except calls for help) and not spam the message box with zombie conversations about brains while player is trying to read the important combat messages. I know relatively good solutions to both, so let's just keep this thread about theming.
Do we want the zombies to talk?
Having zombies occasionally say human stuff while not seeing player around would betray their position in a more flavorful way than having them stumble around loudly all the time.
Typical idle zombie stuff could be just muffled phrases people say to no one in particular ("where did I put...", "what was I doing?", "cold here", "when we eatin'?"), dialogue with people who don't exist ("of course, you see...", "can you turn up the heat?"), mixed with some less normal themes ("lost my eye", "need to murder neighbor").
Aggroed zombies would say different things: sound angry ("I'll fuck you up!", "get out now!"), scared ("mommy help!", "what did I do to you?", "he has a gun!") even while attacking relentlessly, accuse player of being a zombie ("it's one of THEM!", "she just bit me!"), or even sound more friendly than hostile ("excuse me, what's the time?", "hey there sexy", "yo, hold up!")
No actual conversations because that's too hard to implement.
Probably without any simulated muffling. After all, NPCs can speak perfectly clear through gas masks and mi-go parroting likewise can be understood at great range, in the middle of gunfight, by a semi-deaf gunner. It would not make much sense to have zombies say "brainsh" when an illiterate, drugged up mutant with mouth tentacles and a gas mask speaks perfectly clear English. Cutting start/end would be better than muffling.
But talking zombies would be rather weird. We could just have them make sounds: moan, groan, bang their chests (hulks need to do that), gurgle and scream.
Screaming zombies that occasionally say something would also work.
Then there is the third option: zombies as "death from the stars". Just alien entities, no human behaviors left, more machines than animals. Those would only shriek if their role is shrieking.