If you have a web page with a link to the file, just copy the URL into a post.
If by 'stored' you mean you have an account to which you can upload files for storage and that storage space can be publicly navigated, then again just paste the URL.
If none of the above, you'll have to either make a web page with a link to the file and post that URL or you could attach it to a post using the 'manage attachments' button you see at the bottom of a new post. The issue with attaching it is .mp3 doesn't appear to be a supported format so you'd have to zip it first.