Imitating accents has been popular since the '60s. While British singers often have tried to sound American, American pop-punk vocalists mimic the Brits, as well.
For the newest James Bond movie, "Skyfall," English singer Adele recorded a song with the same name. Though Adele speaks with a strong London accent, her singing voice sounds more American than British. Why do British vocalists often sound American when they sing?
Because that's the way everyone expects pop and rock musicians to sound. British pop singers have been imitating American pronunciations since Cliff Richard, the Beatles and the Rolling Stones began recording in the 1960s.
These musicians were largely influenced by the African-American vernacular English of black American blues and rock 'n' roll singers like Chuck Berry, but their faux-American dialects usually comprised aspects of several American dialects. Imitating an American accent involved both the adoption of American vowel sounds and rhoticity: the pronunciation of r's wherever they appear in a word. (Non-rhoticity, by contrast, is the habit of dropping r's at the end of a syllable, as most dialects of England do.) Sometimes Brits attempting to sing in an American style went overboard with the r's, as did Paul McCartney in his cover of "Till There Was You," pronouncing "saw" more like "sawr."
Linguist Peter Trudgill tracked rhoticity in British rock music over the years and found that the Beatles' pronunciation of r's decreased over the course of the 1960s, settling into a transatlantic sound that incorporated aspects of both British and American dialects.
The trend also went in the opposite direction as new genres developed: American pop-punk vocalists like Billie Joe Armstrong of Green Day took on a British-tinged accent to sound more like seminal artists such as Joe Strummer of the Clash.
Contemporary singers continue to adopt various accents according to their genre; Keith Urban, who is Australian, sings country music with a marked American Southern accent. A recent study suggests that the default singing accent for New Zealand pop singers uses American vowel sounds, even when the singers aren't trying to sound American, perhaps because today's singers were brought up listening to American (and imitation-American) pop vocals.
Even when singers aren't trying to imitate a particular vocal style associated with a genre, regional dialects tend to get lost in song: Intonation is superseded by melody, vowel length by the duration of each note, and vocal cadences by a song's rhythm. This makes vowel sounds and rhoticity all the more important in conveying accent in song.