Estrella Morente, Sadler's Wells Flamenco Festival

Every February the Sadler’s Wells flamenco festival summons the illusion of Spanish sun onto our chilled, grateful backs - this year singers are getting almost as much prominence as dancers. But what sun, I ask, at Estrella Morente’s dark, often remote evening, opening the fortnight last night? (And why, still, after years of urgent requests, no subtitles for these pungently melodramatic lyrics?)

Morente has a long record despite her young age - she’s only 30 - and was Penélope Cruz’s singing voice in the 2006 Almodóvar film Volver. A past visitor to Sadler's Wells, she is the child of flamenco parents, fêted as a young moderniser of a traditional form. This is a question of babies and bathwater. On the YouTubes her voice and the electronic mixing of her instruments can make an intriguing, haunting effect. On stage last night, she seemed to hang onto her microphone, singing into it, the voice wanting in richness, rather than aiming into the live space vibrating between us and herself. In this she is only following a long new tradition of rock singers who put technology between them and the paying public, but it kills the spell that the old flamenco stars could cast.

Or was it that this one-night London stand came too close to the death last December of her father, himself a celebrated flamenco singer? All round me last night an intent, silent audience concentrated on the young woman, a few of them crying out in Spanish from time to time. I loved the Arabic coils of the songs, the cries that took me back to the muezzins of my childhood, and the unpredictabilities of those teasing rhythms, but I could only admire her, unable to cross the curtain that she drew between her singing and us with her microphone, as she made her relationship with the enormous loudspeakers and the sound equipment, rather than with each of her listeners in a small theatre with fabulous unaided acoustics.

I once went to a Kylie concert - I have never seen such an unforced relationship between a singer and seemingly every last person in the 30,000-packed arena. This is the tension, the purity of the living relationship between singer and listener, and flamenco should be this bewitching last bastion of live engagement with the earth, fire and the vast night sky. Yet now it seems to see its modernising mission as a duty to impose a close, windowless rock-club atmosphere that suffocates the outdoors original, putting the mike sound before the live experience. What doesn’t for a second make up for this loss is the mutual admiration society of constant onstage “olés!” Never did Morente or her three backing male singers draw breath without two or three encouraging shouts from their colleagues. Every other second. You want to hear what the guitar is noodling while the singer breathes in? Forget it. “Olé! Olé!”

It's curious, as if live performance is something to be tackled only with friends and equipment around you. Morente has a strong voice which she uses with good rhythmic flexibility, but listening later to snatches of recording, I found the voice sounded in the theatre quite clean and monochrome, rustic without much vibrato (rather as if the early-music brigade had scrubbed out the aged, wrinkled sobs and throbs of the traditional flamenco voice). She relies on intimacy with the mike to produce tonal variety, rather than getting intimate with us and using her direct sense of our reactions to produce expressiveness. Hence the lack of tonal gradation (though last night one of the speakers wasn’t playing ball, so the sound kept cutting out from the left-hand speaker - it's the risk the mike-dependent singer runs).

She adorned herself in two images, strict governess and bed-bound lover - a shoulder-padded black suit with a long skirt to begin with, followed by a red flounced garment worn with loose hair, giving the impression of a woman having flirtatiously grabbed a frilly counterpane to cover herself. It was in the governess section that after a series of songs with just her and her guitarist, the highly accomplished José Carbonell (known as Montoyita), Morente shed the self-conscious pose and gave us some truth about singing one’s heart out in those alluringly arrhythmic Arabian melismas.

Listen to Estrella Morente singing (wonderfully) "Los Pastores"

Spanish guitar is a tireless plucking of busy fingers, and Montoyita’s fingers are never still, while the voice takes lungfuls of air and spins them out from fortissimo opening vowels to a fast decline into threadbare last notes, and this contrast of motors makes the guitar/voice pairing more fascinating in flamenco than any of the larger ensembles. By the third song, my heart was finally tugged in - I no longer found Morente’s voice over-rhetorical, or Carbonell’s fingers admirably over-occupied. I surrendered, and I felt it was because the two did at last forget about being Names in the music world, and became individuals simply expressing an unfeigned hope that they were doing the heartstopping song justice.

I wish I could tell you which of these songs it was, but it began with a devastatingly long-held cry of “Aiee-owww” that went on and on, vocal arabesques curling and wriggling as if in utmost pain over the restless, surging guitar. She could have been crying out to a country from a tower, a female muezzin calling to arms, or a woman screaming after a man who abandoned her. In that song time and place dissolved, and the rest of the mannered evening had its redemption.

• The Sadler's Wells Flamenco Festival (ends 19 February) continues tonight with dancer Israel Galván, and includes singer Miguel Poveda, Compañía Aída Gómez, Compañía Rocío Molina, guitarist Tomatito and Ballet Flamenco Eva Yerbabuena