What an interesting question. I don't think it does. Singers - such as Lily Allen - who make a point of singing in a natural-sounding London accent, seem to retain their 'glo'al' stops even when singing.

It depends on what kind of singing you're talking about. A classically trained singer generally hits every consonant extremely clearly EXCEPT for "r"s (these, even for Americans, get de-emphasized when not at the beginning of a word), and vowels get cleared up. The result of this is that someone with a really strong regional accent can start singing and a native speaker will have no idea where they're from. In fact, sometimes I'll be surprised if I see an interview with a an I like and discover that they're British or American or from somewhere with a very recognizable accent xD. That said, many singers choose stylistically to use their regular pronunciation in singing which, while it doesn't really work for classical stuff, works wonderfully for lots of other genres such as folk, rock, blues, etc.

Wow, what a great question. The pronunciation of English in singing is hugely VARIABLE depending on the period and singing style and I'm not an expert. English isn't a very "singable" language and it is USUAL for "trained" singers to modify consonants and vowels in many ways. Furthermore, singers use individualistic pronunciation as an expression of personality. I've noticed that in Broadway musicals--miked, but the traditional singing style is intended to project and to be heard acoustically--singers hit every consonant HARD, loud, and distinctly, even more so than in "careful" speech. I'm not a phonologist and am not sure exactly what I'm hearing here, but I think I'm hearing a true "T" in the word "better," particular from Merman. What do you think you hear? "Anything You Can Do, I Can Do Better" Ethel Merman-- https://www.youtube.com/watch?v=I-bgYxMO5cY Bernadette Peters-- https://www.youtube.com/watch?v=7tCd_sviBNY I think I hear a true T in the word "bottle" in this 1960s vocal jazz ensemble https://www.youtube.com/watch?v=EJnQoi8DSE8 And I think it hear it in some but not all occurrence of the word "mountain" in this 1970s pop-folk classic: John Denver, Rocky Mountain High https://www.youtube.com/watch?v=eOB4VdlkzO4 DEFINITELY here with country singer Johnny Cash singing a folk song: "The Big Rock Candy Mountain" https://www.youtube.com/watch?v=khGmp7MJ2MI So my OBSERVATION is that in singing, the need for clear diction outweighs the wish to sound natural, and there is a tendency to pronounce the T fully--even in "folksy" material that is using an informal English style.