I guess I'm just pragmatic about these things, but I find it difficult to think of any sort remix, mash-up or cover that could be done that would make someone a

I guess I'm just pragmatic about these things, but I find it difficult to think of any sort remix, mash-up or cover that could be done that would make someone an "artistic wrecker". Even your example of belching on top of "Yesterday", while not palatable to most people, perhaps, is still definitely art. It comes down to a matter of taste, not a matter of art. I would argue that this isn't even a particularly recent idea or something unique to music. See: Marcel Duchamp's Fountain (1917). As far as it being theft: nobody is going to buy Belching Yesterday under the illusion that they are getting a Beatles recording. There is no lost sale. There may be a possible copyright claim, but I don't think even this is completely cut and dry. Belching Yesterday could win in court as a fair use. You never know...