Astronomers usually pronounce these as:
Pleiades would be pronounced: .....Plee ah dez....; "Plee" as in 'ple-ase', "ah" as in 'a-bout' and 'dez' has a 'long-e' sound not 'short-e'
Hyades would be pronounced ....High ah deez....rhyming with Pleiades.
I have heard other pronounciations like Ply..a..dez and Hay..a..deez but I do not hear these professionally. Perhaps some etomologist 'out there' might know the history better!