Google has apparently added an MP3 playing feature to Gmail. So, if someone emails you an mp3 as an attachment, the little google player will come up to allow you to play the sound right from Gmail. Very neat.
You can also embed this player in your website so your visitors can listen to an audio clip right from any page. The embedded player at Gmail calls this URL to play the attached MP3:
UPDATE: iSpider goes into more detail on how to embed the player in a website.