I've been thinking about this for a while and it seemed pretty easy, but I wasn't sure.  Wanted to know if I could embed text messages in a wav file that someone else could read without discernible artifacts.  Turns out it was pretty easy...
here's a vid

A similar technique can be used to "watermark" an audio file. For example, if you have a sample library you can tell if someone has stolen it and posted it on some Russian warez site under a different name. If it's done right, the tag will survive even after being processed.

Yeah ive played around with adding visual ones using audition's spectral editor. I've heard these techniques are almost to the point the watermarks can be found enen if youre using, say a drum loop in an audio mix. I don't really get how thats possible, but It's a cool technique. I was just thinking this was so easy it couldn't possibly work.

Paul is a dead man, miss him miss him

LOL

You could try something like that clever Aphex Twin fellow and embed an image in the wav which is only viewable with a spectrograph...
http://www.bastwood.com/?page_id=10

yeah that one was awesome - richard's musical style lends itself to visual mangling
a couple of my favorite free tools for that kind of thing are audiopaint and sonic visualizer.  demo vid