Pass fileurl parameter into shortcode from Advanced Custom Fields

Do not use echo in your shortcode. Use return instead.

function get_generic_demo() {
    //Get the field's value
    $demo = get_field( 'generic_demo');
    //Save the URL into a variable
    $url = $demo['url'];
    // Pass it to the other shortcode and return its value
    return do_shortcode('[sc_embed_player fileurl="'.$url.'"]');
}
add_shortcode( 'generic_demo', 'get_generic_demo');

Also make sure your fields contain the proper URL format. Dump the variable by using var_dump to verify it. This is most likely where your problem is coming from.