X-Bloc Reunion, Barbican, London

Voices of rare power and beauty

If Stalin created world music - as the record-producer Joe Boyd argued recently in The Independent on Sunday's Talk of the Town magazine - those who deposed his successors are triumphantly curating his musical legacy. The idea under-lying the Barbican's X-Bloc Reunion is both simple and inspired: to pull together all the strands of the Soviet empire's indigenous musics - from the austerely traditional to the most liberated post-punk rock - and let them ferment together in a 10-day festival. Most of us know the mysterious sound of the Voix Bulgares, and the vogue for throat-singers has put Yat-Kha on many people's musical map, but there are hundreds of other, no less colourful styles to be heard across that vast swathe of the globe.

It was nice that things kicked off not with an easy crowd-pleaser but with one of the Orient's most venerable forms. Azerbaijani mugham singing demands a long apprenticeship - and an audience willing to wait for its fireworks - but with Alim Qasimov we were in the hands of a supreme master. He served his apprenticeship in 20 years of weddings, where he learnt to strike the perfect balance between the requirements of the guests and the high refinements of his art.

After a spike fiddler had regaled us with an ornate solo, Qasimov's frame drum seemed to fly about of its own accord. Putting it aside, and clapping one hand to his ear, he raised his other hand aloft and launched into song, rocking gently from side to side. Impossible to describe his music's turbulent flight or the tracery it wove with the other instruments on stage. Next came an interlude in which two Russians experimented like research scientists on their exotic folk instruments. Then we got Armenia's serene answer to its traditionally hostile neighbour Azerbaijan, in the form of a trio of duduk-players led by Djivan Gasparyan. No instrument could be simpler than this homely piece of apricot wood; none has a more plaintively reedy sound. For me, the trio could have gone on all night, though their hugely puffed-out cheeks indicated what wind power they had to push through these unassuming eight-holed tubes.

The Eastern Divas concert raised awkward questions about how Eastern music should be packaged for the West. Uzbekistan's little-girl superstar Sevara opened with a lovely a cappella folk song imbued with an almost Celtic wistfulness, but the act that ensued was pure MTV: posturing, finger-snapping, bottom-wiggling, working the audience as though they were pill-popping teenage halfwits.

Next up was Tuva's Sainkho Namchylak, the darling of the electronic avant-garde and - like many other singers from her remote region - possessor of a fabulously versatile voice. But, despite moments of beauty, wit and unearthly spookiness, her act was really just the unpacking of a box of vocal tricks. One longed to hear her clean, pure and unamplified.

But with the Gypsy diva Esma Redzepova, we got the real thing. Her Macedonian band were stunning - I've never before seen a clarinettist and a drummer swap instruments and continue their high-wire act without the slightest drop in virtuosity. Attending her as though she were a queen bee, they turned each song into a miniature drama. A raunchy comedy, a seduction in silk, a piercing lament from under a black veil - everything carried irresistible authority. But her voice was over-amplified, too - which was sad, and also unnecessary, with such a magnificent instrument.

Tonight and tomorrow, the festival continues with Bulgaria's Koutev Ensemble, which brings us back to Stalin. Philip Koutev was a Bulgarian musicologist invited by the Soviets in 1949 to form a female choir. He and his wife collected songs and dances in the countryside, which he then set as formal compositions. They were the original source of that powerful, vibrato-free timbre that galvanised Western music-lovers as Le Mystère des Voix Bulgares. Today's exponents still possess it in full measure.

The festival continues until Saturday (020-7638 8891)