Matthew McConaughey would be my pick for the voice/Dormmamu. Carey Mulligan to play Cleo. The novelization to be written by our own Greg Cox! Yes. This must happen.
I don't think I pictured Dormmamu having a southern drawl while playing the bongo drums, shirtless no less. But here are some voice talent that have done it in the past.
My vote would be Tony Todd or Michael Ironside. If only Orson Wells' and his Unicron voice was still around. Or maybe Jeremy Irons.