> Change any of those bits and not just the sound, but the _perception_ of the sound must change, for at least some listeners
I don't think that's really true. My understanding is that the watermark is imperceptible so one could listen to many different copies of the same audio with different watermarks and not be able to tell the difference between them. I would also guess that a file can be re-encoded many times before the additional audio artifacts become perceptible and that a watermark could survive many re-encoding trips before being successfully obliterated. At that point the quality of the audio itself has probably also been obliterated. The people designing watermarks are also likely aware of how the data is encoded and decoded and so can design features that are most likely to survive many round trips. This reminds me of 56k modems which depended on knowledge of how the analog sound data was encoded to PCM digital at the CO.