I'll also add that I'm skeptical that at least the US is going to treat AI models trained on something as intrinsically creating copyright-infringing derivative works, though I don't know for sure what the EU will do. However, even if one assumes that some jurisdiction does decide to treat models as a derivative work, there's a fairly straightforward way to continue to distribute mods that I would expect should remain legal, and has happened in the past to avoid distributing copyrighted assets: distribute them as a patch against the original work.
It is legal for the end user to modify a copyrighted work that he owns. So if I distribute a patch that takes in Voice Actor X's base-game audio as an input and then takes them as input to generate new ones, well, that's not a legal problem for copyright. Copyright only deals with distribution from one person to another. I can create all the derivative works I want myself -- as the end user -- as long as I don't myself distribute them.
In fact, while it's probably not a very CPU-efficient way to distribute it -- going to waste the world's electricity, do another Bitcoin -- one approach might be to just distribute Tortoise TTS or whatever it is that people are using to generate the audio, as well as any marked-up text to regenerate, then just have the regeneration run on the end-user's computer to generate the mod using the original voice assets. Tortoise TTS has expensive generation, but unlike, say, Stable Diffusion, where the training process requires a lot of computational capacity, has very rapid training time on a new voice. Would be bandwidth-efficient, at any rate.
But point is, that is unquestionably legal, and still winds up in a place where the end user has the mod with the same new voice data on his computer.
And given that, I don't really see the point in trying to prohibit distributing the AI-generated speech files, from the standpoint of someone who is trying to block someone from playing a mod for a game using AI generated voice, because that player is going to wind up in basically the same place regardless of which route they take. It's maybe marginally more-obnoxious to take the full regeneration route, maybe have to run the "regenerate the mod voices" process overnight, but it's not going to generally stop the player from getting and playing the mod.