Did you try using app.launchURL() ?
Also, there's no need to offer a script in exchange for help here.
Hi, sorry, what's this? Could you give me an example? I am not a developer...
It can't be done using only a PDF file - any link to a URL will simply open your web browser, and whatever handler that browser uses when presented with MP3 files will be triggered (download, play, etc.) - nothing in the PDF can override that handler and force a "save" operation.
Instead you'll need to configure your website to force downloads (using HTTP headers) and link to those URLs instead; which is a subject outside this forum.
For security reasons, the ability to save an open PDF is restricted to trusted PDFs and trusted scripts. This is discussed in the Acrobat JS API Reference.