Finnish and Welsh accents for Noldor and Sindar? Italian for the Teleri? Hebrew for the Dwarves?
And I guess Germanic language accents for Beorians and Hadorians?
Awkwardly Tolkien used Welsh to represent the Dunlendish linguistic background in Buckland, though.
RP accents can be for the Ainur, but as superhuman (and superelven) talented linguists, I think they can use the accent associated with whatever language they're speaking at the time. Or RP for Ainu telepathy.
Orc actors would of course have their voices obscured by computer effects to make them sound like rocks grinding or smashing together.