Artists should register their digital signatures along with their copyrighted works so that they can prove this sort of piracy in court. For the artists, this makes the cost of doing business exactly one 'net-connected SOAP server, and probably a domain registration every year, although they may choose to farm all of this out to a service. Since the clients by convention try a random url from the list each time, the artist could set up multiple redundant revenue streams. Even if someone hijacks one of your domains, they can't collect your payments without the correct private key. Without that, they can't respond to the challenge.
Key management is always a concern.
Legal control of private keys controlling the copyright should be explicitly enumerated in writing before each recording session or live performance. The reason multiple keys are used to sign each file is so that one or more may be revoked if necessary. This suggests that the Artist thinks the key may have been compromised. Player software could store revoked keys indefinitely and never trust them again. Some record companies may choose not to play this game.