Invisibility spell states that its subject gains +20 to stealth. This makes it really hard to pinpoint a creature by something other than sight (i.e. hearing).
But what if invisible creature says something?
Does fixed DC of a conversation gets modified by creature's bonus to stealth checks, bringing pinpointing DC from a reasonable 20 (0 base DC to hear conversation +20 pinpointing) to a whooping 40 (0 base DC to hear conversation +20 pinpointing +20 invisibility)? Or does it remain fixed and 20?