Not all text2speech AI requires a promp. And not all Text2Speech is AI. (As hechelion said above)
Loquendo and Balabolka are not AI generated for example.
In the other hand Bark and CoquiTTS are AI.
In the legal issue, i think there is a CC0 voice bank you can use. In XTTS or (CoquiXTTS) you only need a 10s voice sample of the voice you want the characters to have. It can be even your own voice distorted. Or perhaps since you want 'ghostly voices' you can use your own voice distorted in Audacity with a noise background and a bit of echo effect? IDK what's more easy for yah.
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But I do not want to demean my work by forcing myself to add generative AI tag as my project is mostly my effort and assets that are not mine are legally acquired by human artists. I prefer not adding the tag but I want to be honest to players and I am getting mixed signals on the internet if text to speech is generative AI or not.
' <- Share the feeling dude, i am also a pro-AI user. I personally i don't have issues enyoing something with AI generated content. The worl (or internet) is mostly polarized in the issue right now because is a new thing in general. Do what you think is correct and suits you well.Good luck with your project.
:-)