merriam-webster_auto_aud.user.js

// ==UserScript==
// @name           Merriam-Webster auto audio
// @namespace      http://d.hatena.ne.jp/murky-satyr
// @description    s/embed/audio/
// @include        http://www.merriam-webster.com/cgi-bin/audio.pl?*
// ==/UserScript==

var embed = document.getElementsByTagName('embed')[0];
var audio = document.createElement('audio');
audio.src = embed.src;
audio.autoplay = true;
embed.parentNode.replaceChild(audio, embed);