I am not sure what you are trying to accomplish, but from the documentation in the config file on the album folder location, "external" is used when there is no WEB based path to the folder. This means the content of the folder is not visible (directly) on to web browsers. So, any feature that uses the "src" attribute will not work!
But in your case it appears that the folder is in fact visible to web browsers. Is that an artifact of your post? If setting the class to "std" really works why are you attempting to use an external folder?
Anyway, in the case of images, they are not directly loaded from the album folders except for the "full image". The resized image is loaded from the cache folder which is visible to WEB browsers. (The full image gets served from the cache sometimes as well.)
While it might be possible to serve the multi-media file directly via a php script that has not been implemented. First, few users make use of the "external" album class. Second, many "handlers" are sensitive to the file suffix in the link and do not handle the PHP extension.