Looking through some old octavos, I found a score edited by Robert Shaw, often noted as one of the most influential choral conductors of the twentieth century (mainly his work in popularizing the act of singing in a choir). This Shaw, like the similarly named George Bernard Shaw, was interested in breaking apart the sounds of the English language. Both Shaws worked to create a way of accurately depicting the English language’s sounds for a large audience. Bernard Shaw willed a contest to design an ideal alphabet for the English language (which was finished in the sixties and actually is quite attractive and now Unicode supported). He wanted the alphabet to distinguish every sound of the English language, which meant having about 40 to 48 letters.
Robert Shaw’s attempt was more populist. He created a second line of lyrics in some of his earlier choral octavos to try to unify the sounds of a choir. Let’s look at his version of the first verse of “God Rest You Merry Gentlemen”:
Gawdrehsteeoommehree jentuhlmmen, Lehtnuhthing eeoo dĭsmayee. Reemmembuhr Kraheestahwuhr Sayeeveeuhr oouh zbohwuhrnnawn Krismuhsdayee; Too sayeevuhsawl from Sayeetanspahwuhr hooehnnooee oohr gawnnuhstrayee Ooo taheedingzov kuhumfohuhrtandjoee, kumfohuhrtandjoee, Ooo taheedingzov kuhumfohuhrtandjoee.
If I translate this into IPA, what I get is this:
/ɡɒd rɛst i̯u mːɛɹiː ʤɛntəlmɛn lɛt nʌθɪŋ i̯uː dɪsmei riːmɛmbɚ kɹɑist ɑwɚ sejiviɚ uəz bowɚn ɒn kɹɪsməs dei tuː seiv əs ɒl fɹɑm seitænz pɑwɚ hu̯ɛn u̯iː wɚ ɡɒn əstrei oːu tɑidɪŋz ɑv kʌːmfɚt ænd ʤɔi/
It is supposedly US English, but possibly influenced by the cot-caught merger that would have been present in Shaw’s growing up years in northern California, he seems not to be able to hear the difference among /a/, /ɑ/, and /ɒ/. He puts God, and on, and gone on the rounded British vowel /ɒ/ instead of /ɑ/ (in his ears having the same vowel as the word all), tidings and Christ get /ɑ/ instead of /a/, and from gets /ɑ/.
He also conflates the approximants with vowels. For you, he has /i̯uː/ instead of /juː/. For when, he has /hu̯ɛn/ instead of /hwɛn/ or /ʍɛn/.
Most importantly, his choice of the British pronunciation of the short o (in the US, it is /ɑ/ and in the UK it is /ɒ/) probably had some influence on the modern conflation of most of the low back vowels as a single nondescript vowel.
I cannot imagine this kind of thinking started with him, but it certainly did not end with him, as it is still a serious issue today in choirs. I might compare it to Transatlantic English, which was the preferred accent of movies for most of the early to mid twentieth century. By the late twentieth century, it fell out of favor. The choral community just hasn’t caught up with that trend yet.
Shaw is often quoted as saying “When we sing, we don’t sing words, but sounds that sound like words.” While that might be partially true, it is still important to sing the right sounds so that the sounds sound like the right words.