Simple as that - is it possible to make an NPC speaking a dialogue line, but without his lips moving? So that there's an illusion of "voice inside of Player's head", "telepathy-like" dialogue?

You could simply just not generate a lip file. Alternatively, you could try using a talking activator depending on exactly what you're going for.

Well, the idea is - a few "ghosts" in one room, close to each other, which can talk to you telepathycally. The file would be too risky - the Activator would work, if I could set the script to both play the sound and stop player movement for the period of time when the ghost is talking, so that player won't be running around the triggerboxes, causing a spam wave of all the dialogues of every ghost at once.