I've heard the same phenomenon. And as an aficionado of British TV, I've witnessed the British style of speaking evolve over fifty years, too. A lot of it is cultural evolution, and you can hear "foreign" influences creeping into not just the language, but the way of speaking it. American speakers, mostly the young, are prone to slip into a mildly sing-song "urban" style. UK speakers often crack me up with their casual Americanisms, often delivered effortlessly slipping into and out of an American accent.

But you're also right about the influence of technology, and here, of microphones.

When I first joined up with my high school radio station in 1970, we got some training in how to speak on the radio; that is, how to effectively "work" a microphone. Microphones then were good, better than used in those old films, but they were still prone to distortion when hit with certain sounds. We were taught to avoid a puff of air when pronouncing "P", because it overloaded the mic and produced a breathy percussion; tap the lips together instead. The "S" sound was similarly problemmatic, prone to loud hissing, and we were taught to minimize it, even directing the air downward from the mouth and away from the microphone. Generally we were taught to exaggerate the motions of our mouths in service to clearer diction. The end result was the distinctive speech patterns of a radio or TV announcer.

Microphones are much better these days, with none of the problems that required speakers to compensate; and actors and announcers today tend to speak more naturally, and be less self-conscious of having to "work" the microphone.

Another observation I'd make, speaking of microphone technology, is that different shops had different audio standards. The BBC has long had a "Radiophonic Workshop" that developed special sounds and technical standards for audio, and it gives BBC productions their own distinctive sound. Not necessarily for the good: Americans complain they can't understand British actors, and I think it's partly because the BBC audio standards shortchange the high end. British audio has sounded flat for decades, but lately it's a lot more understandable (or maybe I'm just watching enough British TV that I'm finally able to understand their muttering). British actors, too, aren't having to work as hard to compensate for the shortcomings of the audio system.

The focus on technology isn't to say that patterns of speech haven't evolved culturally. Our parents' generation definitely did speak in a more refined, modulated tone, with better grammar and diction. And it came about without constant examples of recorded speech to mimic; they were taught to speak that way in classrooms and at home, for the most part.