I just watched a video that showed how to do this. I think it's pretty cool.
1) Take a song on your computer. It doesn't necessarily have to be an Mp3 file, but I'm not sure if this works with all audio formats.
2) Change the extension from .mp3 to .raw.
3) Open up the .raw file in Photoshop. You'll see a giant square with pixelated blacks, whites, and different shades of gray.
4) Using the Text tool, write a message in black.
5) Save it as a new .raw file.
6) After closing out of Photoshop, change the new file's extension from .raw to .mp3.
7) If you play the mp3, you'll start to hear weird sounds about halfway through.
Now if you have a friend that also knows this trick, you can send secret messages back and forth to each other. Send each other MP3s, and then open them up in Photoshop as RAW files to read the message! If anyone else uses the computer you're on, all they'll find are weird-sounding songs; your messages are safe. Who knows what strange and limitless possibilities you could use this for!