add_extension_to_links.user.js

// ==UserScript==
// @name           soundcloud.com: append extension to download links
// @namespace      http://d.hatena.ne.jp/youpy/
// @include        http://soundcloud.com/*
// @require        http://gist.github.com/raw/3242/1a7950e033a207efcfc233ae8d9939b676bdbf46
// ==/UserScript==

(function() {
  function appendExtension(links) {
    links.forEach(function(link) {
      var format = link.className.match(/filetype (\w+)/)[1];
      link.href = [link.href, format].join('.');
    });
  }

  function links(doc) {
    return $X('//a[@title="Uploaded format"]', doc);
  }

  appendExtension(links(document));
})();