Three other factors (that
may already have been discussed - I'm skimming this thread).
1 If you watch the credits at the end, there will probably be a 'dialog coach'. (The one on the Bernstein recording of West Side Story had a hard job with José Carreras singing the part of Tony - a White Anglo-Saxon Protestant.) Dialog coaches pay more attention to the songs than to the spoken bits.
2 The internal structure of a song (especially rhymes) forces people with regional accents to use a more standard accent. As long as you, as a student, have learnt the standard accent, the songs are likely to seem clearer than the speech.
3 As a choral singer myself, I don't think I've ever been to a rehearsal where the conductor didn't tell us to make the diction clearer (or, much more rarely, praise our diction
) - sometimes they emphasize the importance of diction, sometimes articulation, sometimes pronunciation, sometimes 'underlay', often - especially during a quiet part - they just mouth the word 'Words' or 'consonants'. So people singing will often be making an effort to be clear.
b