The BBC presenter Lyse Doucet, pronounces it LEEZ. She's Canadian ... is that close enough?
Nyx is always said to be a goddess, but there are no surviving stories about her so I rather think she's more a personification. And night has always had rather a bad press! The darkness, the oblivion, the sudden, unexplained terror ... Having said that, I know a charming teenage Nicole whose nickname is Nyx; not sure how she spells it.