A correspondent writes to say he's noted that regional accents disappear in songs, or at least become less detectable, and wonders if there's an explanation.
This is true, as a general observation, and there two reasons for it. The first is phonetic. Several of the main identifying features of a regional accent tend to disappear when singing - the intonation (obviously, as a melody replaces it), the speech rhythm, and vowel length (for many syllables are elongated). Vowel quality is also often affected, especially in classical singing, where vowels are articulated with greater openness than in everyday speech.
All of this can affect the artistry. I found a quote from Billy Bragg saying that a London accent forces a singer to approach melody differently. ‘You can’t sing something like 'Tracks Of Your Tears' in a London accent. The cadences are all wrong. It’s also difficult to sing harmonies in a London accent. And you can’t sustain syllables for long'. It's not possible to generalize from this, because accents have very different norms - different rhythms and rates of articulation, for example - but it's interesting that some singers have reflected on the issues.
The other reason for accent levelling in songs is social. Some singers want to drop their regional accent, because they want to sing like the fashionable mainstream. This has been especially noticeable in popular music since the early days of rock 'n' roll. Singers everywhere imitated Bill Haley and Elvis, and many still do. A mid-Atlantic hybrid quickly emerged, which levelled natural regional features. From his singing, who would ever guess where Cliff Richard comes from? Or Sting, Rod Stewart, Tom Jones, or Elton John?
However, it's perfectly possible for singers to retain an individual accent, if they want to, and many do. In fact, they've been doing it for years. If we listen to recordings of music-hall days, we'll hear broad Cockney, Lancashire, Scots, Irish, and others. You could hardly get more Cockney, for example, than in such songs as 'Any Old Iron' or 'Boiled Beef and Carrots'.
And now there are signs of modern pop music returning to its dialect roots. The Mersey sound was an early development. A Liverpudlian accent regularly stands out in the Beatles - such as (in 'Penny Lane') customer with a rounded first vowel and words like there and wear (in 'Only a Northern Song') with a central vowel (rhyming with her). I recall Paul McCartney saying (but I can't remember where) that the Beatles did experiment with singing in an American accent early on, but decided against it because it sounded ridiculous. Other early departures in the UK from an American-sounding norm (or, at least, a mid-Atlantic-sounding norm) were Tommy Steele and Joe Brown.
More recently we have the London accents of Ian Dury, Chas & Dave, and Lily Allen, and the rather more gentrified tones of Anthony Newley. Mike Skinner's accent is so noticeable (with its glottal stops, replacement of th by f, and other Cockney features) that it has been called Mockney. The accents of the Celtic areas of the British Isles are often heard. Listen for example to 'Daddy's Gone' from Glasvegas and you'll hear several local Scottish features, such as a rounded [y] in you, an [e] vowel in sitting, and plenty of glottal stops. Glottal stops are one of the things to listen out for, actually: you'll hear them in groups from different parts of the UK, such as Futurehead and The Rakes. Listen out too for the /r/ after a vowel in Irish accents, as heard in, say, Mary Coughlan (from the south) and Snow Patrol (from the North). And of course in rapping we regularly get a distinctive accent, because of the syllable-timed rhythm. But my impression is that, rapping aside, in hardly any case do singers use a consistent regional accent throughout the whole song. Mixed accents seem to be the norm.