BRIEFLY: The four-year old World Festival Choir, a non-profit, international choral ensemble which launched itself at a Handel Tricentennial concert in Sweden and appears in configurations of from 3,000 voices down to merely 300-strong, seeks new members. For particulars of upcoming Los Angeles auditions, call (818) 793-2560 . . . The UCLA Piano Competition, which this column mistakenly identified as taking place last week, actually is this week. Semifinals are scheduled in Schoenberg Hall Auditorium Wednesday and Thursday, with the finals to be heard next Sunday from 3 to 6 p.m. in Royce Hall. Information: (213) 825-4761 . . . Meanwhile, in Switzerland, pianist Gustavo Romero, 24, has taken the first prize at the 1989 Clara Haskil competition in Vevey. Romero, a Juilliard graduate who grew up in San Diego, beat out a field of 42 other pianists from 20 nations. His prize is worth $7,500. Runners-up among the six finalists were Patricia Pagny of France and Marietta Petkova of Bulgaria.