sans text sound: deep, short human-like sounds
sans voice headcanon: low and drawling, tired-sounding, pretty accurate to in-game sound
papyrus text sound: even lower than sans, rough and scratchy
papyrus voice headcanon: SKELETOR
to be fair he also speaks in all caps and says NYEH HEH HEH which is recognizable as skeletor and you’ll see most people make that connection very quickly