LARRY London is beginning to learn what it's like to have the biggest radio audience on earth. And we're not referring to the FM Select channel director's mid-morning show in Hongkong. We are talking of a nightly radio audience of almost 100 million listeners for Larry and his Mandarin speaking co-host from Beijing's government-controlled radio, Dai Yi Rong, - and if a DJ anywhere in the world (including ubiquitous US jock Casey Kasem)can lay claim to a bigger audience we would like to hear about it. Every night from 9 pm to 11 pm Larry and his partner present Joy FM which is produced at Metro Broadcast and transmitted by China Radio International (formerly Beijing Radio) to 12 major cities, including Shanghai, Guangzhou and Nanjing. So, anxious to glean a profile of a typical Joy FM listener (demographics is a concept yet to catch on in China), Larry requested on air two weeks ago that listeners write in giving details of the people, places and things that bring them joy. The bulging mail bags have not stopped arriving at Metro Broadcast's Hunghom office since - with more than 6,000 letters received by yesterday. ''I can hardly step into my office'' said a delighted Larry, his desk swamped with neatly addressed envelopes bearing mainland stamps. ''It is quite amazing.'' More than half of the letters are written in English, and having had a chance to read some we were tremendously impressed by the command of English shown by the teenagers and also by their surprisingly sophisticated rock tastes. One Grade 2 student named X. Pung wrote that his favourite song was Patience by Guns N' Roses (so much for Canto-pop!). Larry, with 17 years of radio experience in the United States behind him, said: ''What the letters have brought home to me is how Westernised these young Chinese kids are. They seem to know a lot about English music, which is quite surprising consideringthat until Joy FM came along, there had not been that much by way of Western pop music for them to tune in to. ''It's a vast audience, but the letters certainly give the impression that we are one big family.'' The family connotation is not surprising. Joy FM is produced by Larry's brother Lawrence, who is also a part-time karate instructor. If the idolising tone of some of the letters is any guideline, then Larry will need his brother's expertise to safeguard him should he decide to visit his ''fans'' in China.