The rippling rhythms of a drum corps drifted over Dolores Park from the direction of Mission High School around 8:20 this morning. In the background, voices shouted in unison in what sounded like a military exercise. The serious drums played on. Then came the contrasting magical flow of something like a xylophone or celesta. The voices shouted now and then while the drums and lyrical melody weaved continuously, and the distant performance became extraordinary and moving.