The bonus of Keyring music (from the industry's standpoint) is that you could encode the concert in a proprietary format that would only play on a bundled jukebox. Alternately you could use Microsoft's DRM or iTunes and simply include a unique key which then requires authentication with a server. Dabs sells 64Mb keys for as little as £6. Buy in bulk and you are probably looking at less thana a fiver - add a logo etc... you could probably flog them for as much as £20 each.