Unless you’re reading out loud, the words scrolling through your head are spoken in your own unique, idiosyncratic voice. Pronunciation is at your discretion, an unregulated free-for-all. Any given word can sound exactly as you damn-well please.
Such individual variances can make such things as the pronunciation guides many fantasy authors place in their novels rather superfluous. Sure, sometimes you’ll try, going to the effort of mentally molding some invented name with two hyphens and an umlaut for the first few times you read it. But eventually you lapse into whatever pronunciation seems fitting to your internal editor. I mean really, how many of us go to the trouble of figuring out how Robert E. Howard intended us to say Bêlit? How does a circumflex (that little upward pointing diacritical mark) modify a word? I think it is supposed to make the “e” somewhat elongated, going up in inflection then down, sort of like “Bay-ih-lit.” But I’m not entirely sure.