I think so far as 'voluntary' versus 'involuntary' change is concerned it depends how you want to apply a 'natural selection' analogy. I was thinking of it as the song that was undergoing selection and it wasn't doing anything voluntarily.(and they hate anthropomorphism :-))

For example, it has been suggested that organisms can retain apparantly useless features at times of 'low selection pressure'. If the environment changes there may be a competitive advantage in losing the features or they may turn out to be an advantage. So maybe corruptions in a song could persist for a long time if they didn't bother the audience or singer and there were no preferred versions around. Along comes 1960's vinyl as a potential 'ecological niche' and a singer or producer may decide to tidy it up to suite a wider audience. Conversly, a corruption making something slightly mysterious (Residue, sing residue ...) may make it a succeed with a new audience where a pristine original would not.