[Plugin: NextGEN Gallery] Adding Music-added to media library put url in options slideshows

I am trying to add music for the first time. I added an mp3 file to my media library then I copied the url and pasted it into Gallery, Options, slideshows background music url and I still don’t hear any music. What am I doing wrong?

https://www.ads-software.com/extend/plugins/nextgen-gallery/