It's actually voice technique for the most part. Traditional old microphones weren't as good at recording so actors were specially trained to speak and articulate well to get past the interference and background noise since filtering wasn't really done. As microphones and technological advances improved so did the microphones and the need for special articulation became less and less until somewhere in the 70s or 80s the culture in film shifted to normal voices to have better immersion between the audience and the movie scene. And it just went from there. Nowadays our microphones are so good that even whispered conversations in intimate scenes can be well recorded. So the actors basically just use their everyday voice or try to emulate a real accent which are often slurred.