Okay okay, check this out...
Trait idea, -8 Points.
Occasionally, someone nearby will (from a pre-determined list of phrases/sayings) say something but in actuality, didn't actually say anything at all, it's all in the characters head/spirits.
I like this. In order for it to be sufficiently RP-relevant, the list of phrases should be plausible and include things that should prompt a response, like "Get out of here" or "Follow me" - maybe even threatening ones, accompanied by an illusory hostile intent, like ":snarls" or "I'm coming for you!"
In terms of pricing this, I'd compare it to Deaf which, at the cost of not hearing anything at all, also quite RP-disruptive, costs -4 points. So I picture if the trait is cleverly implemented and has a list of really disruptive, interesting phantom phrases for you to hear, then it might be a -4 at best. But if it's fairly easy to roleplay around, with a limited wordlist that you eventually learn (and can thus gain more skepticism when you hear those phrases) then I'd picture even -2 might be appropriate.
I endorse this idea! Though I could see it as being a -4 or -2 trait, as it could probably be ignored. I can imagine it leading to some very good roleplaying.
If instead of a pre-generated list, it instead pulled from lines of actual dialog spoken elsewhere in the world and fed them to you at sparse intervals, with no context about who said it, it would be less predictable. You might occasionally also get some accidental insight into private conversations, but without the context of who said it or about what, you'd never know how to act on it. Add in a disembodied speech bubble nearby, like when someone hidden is talking to you, and I think it would be great.
I think as a trait this leans a little too far toward gamifying serious mental illness, even if it is flavored as "hearing spirits". As a black magic spell or poison, it might be ok.
I agree with Hellebore, and would prefer having it as a negative effect of magic or poison as well.