The voice shift I most recall was Troi-in season 1(maybe season 2) her accent sounded way more "foreign" and "exotic" than the way Sirtis actually talks. By season 4 or 5 her voice was basically the same as Sirtis voice at convention.
In universe, it could be explained that Troi liked imitating the voice of Mr. Hommn’s predecessor. We have that Season 1 line about the Troi’s butler trying to change Deanna’s accent (that neither of her parents spoke with!)
But for Picardo I think he lowered his voice on purpose. If you watch his “Homr Improvrment” appearances, he has the higher voice.