While I was at the NAB (National Association of Broadcasters) show last year, I came across a company in the radio hall that had developed a way to mark audio so that if it was copied, it could be traced back to its original source. Sampling has made this a significant problem in the recording information. The person that I talked to did not say or seem to know much about the actual process used, but claimed that the signature could withstand sampling and manipulation. I don't remember the name of the company, but I think the name started with cyrpto Roy