Reader Jared wrote in with this question: "Why do singers I perceive as having accents (i.e. Adele, Bono, etc.) have those accents when they talk, but not when they sing?"
I hear what Jared is saying. Or, rather, I don't hear it. While there are certainly exceptions, I've heard a thick accent on many European singers when they give interviews, but they sound as American as apple pie - which, to American ears, means "no" accent - when belting out their songs. (Except maybe these guys.) If I'd only heard Eric Clapton or Bono sing instead of speak, I'd believe you if you said they were from the States.
There are two main reasons, from what I can tell, for this perceived loss of accent.
One is technical. As Billy Bragg — a guy who's never had difficulty letting his accent shine through — explains, "You can’t sing something like 'Tracks of Your Tears' in a London accent. The cadences are all wrong." Different accents are often defined by their rhythms, intonation and vowel quality and length. For many accents, the tune and the rhythm of a song can constrain these qualities to the point where the accent seemingly disappears.
This is true even for certain qualities of the General American accent and regional American accents related to it. GenAm is a rhotic accent, which means speakers pronounce the letter r at the end of words like car and lover. But if most Americans sang those words the same way they said them, they'd sound like pirates. Instead, many songs force American singers to push the r more towards a vowel ah sound, the same way many Brits might pronounce it. (See Jackie Wilson's "Higher and Higher" for examples of both sounds. In some performances he pronounces the r fully, and in others he holds back on it.)
Of course, it is possible for a variety of accents to maintain their unique characteristics within the constraints of song. There's no mistaking where The Beatles, The Proclaimers or The Pogues were from. So, if you can sing with your accent, why wouldn't you?
There also seems to be a social factor to the Incredible Disappearing Accent. I'm just speculating here, but if they have a very thick regional or working-class accent, some singers may want to drop it on their way to music superstardom in favor of a more fashionable or mainstream accent (the exception being American country and western music, where a down home Southern drawl gets you major street cred). Still others might have masked their accent's particular eccentricities in an effort to imitate the sound of their musical idols. This might help explain why the "British Invasion" bands, whose appeal to Americans was their very Britishness, largely kept their accents in their songs, but acts like Led Zeppelin, Cream and the Rolling Stones, heavily influence by African American blues musicians, had more American-sounding vocals.
* * * *
While we're on the topic, here's a question I'd love to see some input on in the comments: Is this apparent accent loss peculiar to foreign ears? That is, can Brits still detect the accent on Adele when Americans can't? Do foreign ears just miss the subtleties of Bono's accent still shining through when he sings? What about the differences in two regional accents? In that vein, even though I'm not from the South, I listen to a lot of country music, and can tell the difference between a singer from Texas and one from Tennessee. To my friends who aren't country fans, however, they both just sound "Southern."