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 feature was initially used only for the voicemails sent from Google Talk. Google uses a Flash player, similar to the one from Google Video.
You can create a new filter for MP3 files, so it’s easier to find them. You need to add a new label.