Anonymized voice recordings? I can't recall if the last article that I read about that a while ago mentioned it, but either way, I would say that it is pretty much impossible to completely anonymize a voice recording.

Nor would anonymising a voice recording (presumably by doing some sort of frequency-domain transform) be of much use to them for the sort of thing they're using them for - training speech recognition on a wide variety of different voices.

It's virtually impossible to anonymize voice. Except by doing voice to text, and then synthesize voice.

Even if not "completely", there is a difference to me as a consumer

The data has a common ID, but that ID isn't linked to your Apple ID, and you can change the ID at any time.