It may have been the first audience in history garbed completely in 100% wool. Dutch composer Clemens van der Ven and his quartet performed his avant-garde piece "Woolstock" to an audience of 270 sheep over the weekend--but the assemblage was mostly unresponsive. Van der Ven, a 37-year-old drummer who says he can't read a note of music, said the idea for the piece came to him three years ago when he stunned a herd of sheep into silence with his singing during a visit to his brother's home in the Dutch countryside. Instruments used in "Woolstock" included drums, a Celtic harp, a Japanese flute known as a shakuhachi and a didjeridu , a hollowed-out tree trunk favored as a musical instrument by the Australian aborigines . . . nothing, it should be noted, employing lamb's-gut strings.