Hey guys, for a lecture I need to listen to a .mp3 file that is embedded on a website. My macbook pro (snow leapord) wont play it. Any software you guys konw that will?

Try this :

Open iTunes -> Go to iTunes preferences -> Select the "Advanced" tab ->Find "Use iTunes for internet playback" -> Click the "Set" button next to it.

When you say "won't play" do you mean the browser plugin doesn't load (error symbol) or that it will load and won't actually produce any sound?

Typically the QuickTime plugin will play any embedded mp3 so long as it's actually an mp3, wav, or other standard (mainstream) audio format.