Why is it that some singers manage to pronounce words clearly when they are singing? and others do not. Opera singers are notoriously hard to understand: something I suppose to do with singing very loudly. Wide vibrato doesn't help, and singers have a lot of vibrato. I blame large auditoria.
Keeping the line, while letting the lips, teeth and the tip of the tongue, freely articulate the consonants, is hard to learn. Consonants are common to all languages, but vowels vary a lot. The curse of English is the diphthong. Without a good legato (which for singers means getting the vowels to have the same timbre), you are sunk.
One of the best singers for English diction is Roderick Williams: when he paid a visit to Stamford Arts Centre a while back, it was a joy to listen to. Every word was clear, even if one didn't know the songs, and one didn't know the songs by Robert Saxton. No-one knew those, but we were engaged nonetheless.
More recently, there was another example of enviable enunciation in singing from soprano Alison Hill, when she and Yair Avidor (see picture) gave a recital of lute songs, A Musical Dreame, at North Luffenham Church, including music by John Dowland, Robert Johnson & Henry Purcell. There is nowhere to hide in lute songs: her line was beautiful, and every word absolutely clear.
Over-enunciation is also a sin. A friend once described what he thought was Ian Bostridge's precious over-pronunciation in Winterreise (in German, on CD) as "more tea, Vicar?". Sadly, I agreed with him.
One of my Desert Island discs is Wilfred Brown singing Dies Natalis - almost perfect. Perhaps his clipped Cambridge enunciation sounds affected nowadays. The recording is also let down by some scrappy orchestral playing, but I can live with the orchestra, to hear Brown in top form.