I can't be arsed to sign in to the online OED but my most recent paper edition gives lay-bee-uh for the plural (in both cases the emphasis is on the first syllable). It'd be interesting to look the word up in a standard medical dictionary which gives Latinate pronunciations though because, if it's not an Americanisation, a change in pronunciation might reflect a shift in the balance of popular usage from the conversational (where I reckon slang is more likely) ro the medical (because women are now allowed to discuss their anatomy with professionals without resorting to euphemisms).

I ♥ phonetic renderings.