It seems like I'm announcing some new Gmail feature every month, but they are definitely welcomed features. This month Google adds the ability to play mp3 audio files attached in emails. This feature adds-on to the recent news of the updated version of Google Talk which allows for voicemail messages; now gTalk users can listen to their voicemail messages within Gmail.
Similar to how you can view the original size of an attached image or simply download it, Gmail gives you the option of playing or downloading attached mp3 files. Playing an mp3 file pops up a window with a basic audio player with a GUI similar to that of the Google Video player.
Update: You can also embed the audio player by calling something like the following (as mentioned on Uneasy Silence):
<iframe src="http://mail.google.com/mail/html/audio.swf?audioUrl=MP3_URL" style="border: 1px solid #aaa;width: 500px; height: 26px;"></iframe>