I say mix it up. It might be funny to have mostly retro sound FX and then occasionally have something all fancy. Sort of an inside joke. And hey, lots of retro games played with sampled voice, so a few samples here and there could be interesting. Maybe a selection of sampled things at the beginning of each level. Like a mummy threatening to curse your intrepid explorer. Display the level and pause it while there's an ominous "Who dares disturb my tomb?!" A few levels later you could hear "I curse you, grave robber!" or something like that. Play with it, yo!
I can do nice, deep, ominous voices well, so if you need some samples, lemme know.
