The Emperor doesn't really articulate well in Episode III, he just mumbled and somehow his voice was clear, and I always thought it was that way because he just underwent lightning surgery. By RotJ he apparently got used to it and speaks just fine.

In ESB though, with the DVD hologram, he looks just like Episode III Palpy. Adywan corrected this by using the RotJ face and the normal mouth, but that doesn't help his speaking problems. What, did he go to a speech therapist between V and VI?