get attachment url for already uploaded images (media library)

Hello,
I try to insert a tumblr button to let visitors share images from my posts.
I do not use featured images, so the trick with
wp_get_attachment_image_src ( get_post_thumbnail_id ( $post->ID ), 'full' )
does not work. So I use the following code:

$args = array(
	'post_type' => 'attachment',
	'posts_per_page' => -1,
	'post_status' =>'any',
	'post_parent' => $post->ID
);
$attachments = get_posts( $args );
$img_url="";
if ( $attachments ) {
	foreach ( $attachments as $attachment ) {
		$img_url=wp_get_attachment_url( $attachment->ID );
	}
}

This works so far for all images that I upload directly when creating the post, but is does not work for images hat I choose from the media library.

How can I get the url of those files, too?

Thanks for hints!

Daniel