Gmail has a built-in player for MP3 attachments. If you receive emails that include MP3 files (maybe a podcast or a public domain recording), you can listen them directly from Gmail. This is similar to the player used for voicemails from Google Talk.
You can create a new filter for MP3 files, so it's easier to find them. You need to add a new label. You can check how it works, just append the following URL of an MP3 file to:
http://mail.google.com/mail/html/audio.swf?audioUrl=LINK TO YOUR MP3
If you want to add Gmail player (like above) to your site just paste the following code:
<iframe src="URL to MP3 File -like e.g. above" style="width: 500px; height: 25px; border: 1px solid #aaa;" id="musicPlayer"></iframe>
You can also replace above podcast with an MP3 URL of your choice: