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.
