To modify the “Speak your mind” text, add the following code to your functions.php file and adjust it to say whatever you want. (edit the part that says “Leave a comment”)
/** Modify the speak your mind text */
add_filter( 'genesis_comment_form_args', 'custom_comment_form_args' );
function custom_comment_form_args($args) {
$args['title_reply'] = 'Leave a Comment';
return $args;
}