You indeed have raised a very interesting question I always have wanted to ask myself. I once asked several young people from the US and the UK whether or not they can distinguish the accent a pop singer has and the answers were very diverse. It turned out that it is not always possible to determine the accent of a singer because articulation is different when a person speaks and when he sings. When you listen to Bono of U2 singing you can not always make out his Irish accent whereas when he speaks you cleary can especially because of the distinctive Irish intonation. When you hear Jon Bon Jovi sing you can clearly hear that he is American whereas when you listen to Madonna it is is not that obvious. What do you think?