CamLangSci, the blog of linguistics students at the University of Cambridge, has a great overview of the classic study comparing the speech of the Queen of England during her Christmas broadcasts across different years:
So what exactly has changed from the 1950s to the 1980s? If you listen to the two YouTube clips below, you’ll notice a wealth of interesting phonetic phenomena. For instance, in the clip from 1957, notice how she says the word “often” (/ɔːfən/, or orfen, around 0:55 in the clip), whereas in the 1987 Christmas message has her saying something closer to /ɒfən/ (or ofen) in the word “often” (at 2:33).
Similarly, in the early clip her /uː/ vowel in “you” and “too” is very back, whereas in the later clip it’s more fronted, that is, closer to the vowel the rest of us are likely to produce. Another interesting feature to look out for is the second vowel in the word “historY”, which is produced like the /ə/ in sofA in the early clip (at 1:22), but closer to the /ɪ/ vowel in the word “kit” in the later clip (in words like “expectEd”).