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//If you have 'search.php' add this to the top of the script, if not put it in 'index.php'

global $wp_query;
if(is_search()) :
    $cat = array(
        'category__in' => array(5) // Where 5 is the ID of your Music category
    );
$args = array_merge( $wp_query->query, $cat );
endif;
query_post($args);