How to get the mp3 or the URL (file) of the current article to attach to the custom button

Please, how can I get the URL of the mp3 file so that I can attach it to a custom button that I have done. I was able to get the title of the message, but I can not do it for the URL of the file.
I want that when I click on the download button the download starts immediately not taking me to the post office.
See my current code and check the screen capture also or the url of the site

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I forgot to add my current codes. Please forgive me if I can not ask the question. see my current code below