Some 300 Jackson Elementary violinists joined Maestro Joseph Silverstein in making their instruments sing last week. Every student at the Salt Lake school learns to play the violin beginning in the third grade as part of a grant to study the effects of music on children. The Jackson students have played for Silverstein over the past five years. "He just does it out of the goodness of his heart," said music teacher Trish Wade. "It's fun to see the power music can have on kids - they just glow when they play for him." Silverstein will pass the baton to Keith Lockhart this fall after 15 years at the helm of the Utah Symphony. Silverstein, whose music career has spanned almost 50 years, is conducting his final concerts this month. The children played for him at the Symphony Guild celebration last week.