How do I make an MP3 file, which is located on the server, downloadable?

if you have a free site you can't have mp3's on your site

you would have to host it elsewhere and provide a link to it on your site


if you have paid hosting etc.. you could put it on your site and make a link to it for people to download

might be best to find somewhere else to host it to save you bandwidth

Assuming that you have a hosting contract that doesn't restrict the file types that you can upload then:
1. zip it
2. upload it
3. link to it in the normal way

A zip file will always download. If you leave it in MP3 format then the visitors PC may attempt to play it rather than let them save it depending on how they have configured their browser.