By Michael Roddy TUNSTALL, England (Reuters) - The voices of 650 youngsters singing Benjamin Britten's "Friday Afternoons" song cycle filled the concert hall he built in eastern England and wafted out over the Internet on Friday as the world celebrated the centenary of Britain's most famous modern composer. Some 100,000 schoolchildren singing in locations across Britain were hooked up via the web with performances of the same songs by children who kicked off the birthday celebration in Melbourne, Australia, at 0300 GMT and were to bring down the curtain in Santa Monica, California, at 2200 GMT. The various versions of the cycle were posted on a website set up by Aldeburgh Music, which runs the Snape Maltings hall Britten founded. The BBC will play Britten's music live and from recordings all weekend on its classical music station Radio 3.