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TANGLEWOOD OPENS ITS 50TH SEASON tonight with flutist James Galway joining the Boston Symphony Orchestra, conducted by Seiji Ozawa. Volunteers and Tanglewood Music Center students, holding 1,000 candles, will surround the outdoor shed during a special performance by the Tanglewood Festival Chorus. ``Happy Birthday'' will be among the offerings, and more than 10,000 cakes will be served. Leontyne Price sings with the BSO tomorrow night, and Charles Dutoit conducts an all-Beethoven program on Sunday. THE LIFE AND MUSIC OF LOUIS (SATCHMO) ARMSTRONG will be portrayed in an international touring musical, ``Satchmo: America's Musical Legend,'' written and directed by Jerry Bilik. Bryon Stripling, a trumpet player from Atlanta, will play Armstrong. ``Satchmo'' previews today in New Orleans at the Theatre of the Performing Arts and plays there through July 19. Other performances will be in Atlanta (July 22-26), Charlotte, N.C. (July 28-Aug. 2), and a five-week engagement at the Kennedy Center in Washington. (Aug. 5). THE FRANKFURT BALLET MAKES ITS AMERICAN DEBUT at the PepsiCo Summerfare in Purchase, N.Y., on July 10, the first day of the festival. They have changed from a classical opera ballet company to an advanced ballet company with a new repertoire and an increasingly international reputation. The engagement features four premi`eres by director William Forsythe. MICHAEL BENNETT, the dancer, choreographer, and director who died yesterday in Tucson, Ariz., will be remembered as the creative force behind ``A Chorus Line,'' the longest-running musical in Broadway history. He got his start as choreographer for ``A Joyful Noise,'' and then went on to create the dances for ``Promises, Promises,'' ``Company,'' and ``Follies.'' His 1981 hit ``Dreamgirls'' just returned to Broadway. He was slated to direct the London musical ``Chess,'' before ill health forced him to bow out in 1986.