Perhaps I’m naive, but I’m not sure that I’m convinced that all of the accent equalization (sounding “American”) is intentional, although the Beatles apparently did this for marketing purposes. I believe, as Grant also said, that the language of singing music has certain qualities which tend to override spoken language (intonation, stress, rhythm, patterns…) Now, “I’m Henery the Eighth, I am…” sounds quite British - intentionally so, or naturally so? Perhaps we need to ask those singers whose native accents disappear while singing if they are cognizant of this change.
An interesting note - Even though Germans love jazz, the language itself is not known for its jazz repetoire. However, there is a jazz singer from Mannheim, Germany, which has its own distinct local dialect, who sings jazz in this dialect and not in standard German. She was quite popular in the 80’s and accepted as a true jazz artist. I believe her name is Jan Fleming. This says something about music’s effect on language, and also about the relationship between a gutsy, native dialect and the nature of jazz. A very interesting topic indeed, further exploration of which would shed light not only on the nature of language, but also of music.