I've started turning on the Subtitles, as it seems as if newer shows and movies are harder to understand. At first, I thought my hearing is going, but not only can I hear the music in the shows just fine, I can watch old shows and movies from over 10 years ago and understand them just fine. In my opinion, it is as if they are putting less volume on the vocal tracks, or maybe using microphones or recording techniques that are not ideal for the spoken language.
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.