Although James Earl Jones has consistently voiced Darth Vader in the movies, there is a difference in his voice between “Star Wars” and “Empire Strikes Back”.

It’s not necessarily night and day, but the more noticeable examples are:

“I want to know what happened to the plans they sent you.”
(https://www.youtube.com/watch?v=2TjiXJiAEn0)

versus

“the force is with you, young Skywalker, but you are not a Jedi yet.”
(https://www.youtube.com/watch?v=igx0nazlpfE)

I’ve wondered if it had to do the with the post-production equipment available or James Earl Jones growing into the role.