I'm listening to a weird concatenation of gongs and bells, underpinned by drums, reflecting a slow and measured beat. It could be a particularly austere new-music improvisation at the Barbican, but it's actually the Thai Elephant Orchestra, whose members have been provided with giant instruments and encouraged to improvise with the aid of their trunks.
It's grave, comic and oddly relaxing to listen to: just one of the mind-opening tracks on The Rough Guide to the Music of Thailand, which hit the shops last week. The highlights include solos on a variety of bamboo instruments, plus vocal numbers such as "Man Motorgai" (motorbike man), which begins: "The bridge of my nose is broken, so why do you love me?" Anyone going to Thailand this summer would be well advised to stick this CD in their bag, as a reminder that there's more to be heard than disco-pap in Phuket.
Rough Guide records are as popular as Rough Guide books and, with 120 titles, cover a comparably vast terrain. Just as the original travel books surprised everyone with their success, so did the CDs. Their begetter, Phil Stanton, started off with just one, The Rough Guide to World Music, which was a bravely pioneering attempt at a 70-minute overview of a seemingly amorphous field. Then he added an African compilation, and it all took off.
That was nine years ago, when it was uphill work to get radio stations, record shops and newspapers to recognise the music of the non-Western world as a worthwhile category. It's easy to forget, given the massive surge in interest over the past two years, how remarkable it was for something as commercially unpromising as The Rough Guide to the Music of Wales to notch up 15,000 sales, and for other series titles to sell 40,000.
"But what really surprised me," Stanton says, "is how the records we thought were risks turned out not to be risks at all." He expected his Canadian record to make a loss, but it soared into profit. He didn't expect much from his Okinawa compilation, but it did well here and sold massively in Japan. "It was partly Japanese who were discovering the music they'd never noticed right under their noses, and partly because it became a hit in Okinawa, where local teenagers saw it as a trendy must-have."
It's ironic that the only failure so far for this ultra-hip outfit has been with hip hop. "We had a great compilation, and we'd spent a lot of money on it, but it was impossible to find out who owned the tracks. All the original companies involved had gone bust, been bought and gone bust again, and we won't do anything unless we know who owns it."
On the other hand, their Delta Blues CD has done well, with Chicago Blues joining it this month in the shops. Sometimes their approach is too voguish for its own good - the Japan compilation dwells so resolutely in clubland that you get very little sense of that country at all - but sometimes they strike a balance that is both unexpected and illuminating, as with the new China compilation this month, or with their foray into belly-dance, which they show to be a great deal more than mere nudge-nudge stuff for the blokes down the pub.
Who buys all this? "An extraordinary cross-section. Doctors, teachers, students, travellers - even people in the armed forces, who want to bone up on the music where they happen to be."
Where will the series end? "It won't. The more you know, the more you know you don't know. I've spent all my life in this area, and I'm still being surprised every day by what I hear," Stanton says.
Rough Guide CDs cost £11: Naxos World's cost £5 and are often just as good. Bhangra Beatz and Bluegrass are their current bestsellers, with Armenian duduk, Bulgarian choral and - believe it or not - Nordic heavy metal coming soon. Naxos's world label manager, Dolores Canavan, claims a 400 per cent increase in sales since the label's relaunch last year, and goes into ecstasies over the Chinese pipa virtuoso Wu Man's CD, From a Distance. "I went to a shop in Brighton last week, and the guy there - who normally specialises in classical - said that every time he plays that pipa CD in his store, he sells it."
Wu Man's CD has been adopted this summer by United Airlines as part of its in-flight entertainment. Yet this is an experimental product by a top Chinese classical musician, who celebrates - rather than betrays - her musical heritage. It's serious stuff.
My next stop is Harmonia Mundi, the French record company that distributes a range of small independent labels, and whose sales director Patrick Lemanski leaves me in no doubt about the way the commercial wind is blowing. Ten years ago, classical music accounted for 80 per cent of their sales: now it's down to 45 per cent, while world music, originally a sideline, has risen to 30 per cent.
What can he tell me about his clientele? He laughs. "We never do market research - that's why we're so successful." The success of Longdistance - a distinguished label Lemanski acquired and re-launched this month - is in his view typical: its bias towards Arabic and Balkan music reflects the passion of the man who created it, and the CDs themselves - I've sampled those from Georgia, Albania and Rajasthan - indicate impeccable musical taste.
Harmonia Mundi CDs are mostly full price, but that's no impediment when you're talking about labels such as Ocora, Wergo, Smithsonian, Institut du Monde Arabe and the superlative Network. No surprise that this latter label's two Desert Blues compilations have become instant best-sellers, or that its Russian, Balkan, and Afghan compilations have become brand leaders: beautifully produced, with substantial liner-notes, they prove that serious ethnomusicology really can slip down a treat.
Which is exactly what I find when I look in on the Tower Records emporium in Piccadilly. "What we've found," says their enthusiastic manager David Acquet, "is that the current surge in interest has led people to look more deeply into the catalogues, beyond obvious things like Buena Vista Social Club and Orchestra Baobab." The bad news for those two embattled giants EMI and Universal is that he and his colleagues are now running promotional campaigns for specialist labels such as Naxos, Navras (Indian classical) and Wrasse (Africa and South America).
"Obviously, a lot of our clientele are tourists," Acquet adds, "but others are regular customers with whom we're on first-name terms, who come in every month and simply take what we recommend."