This seems to have started with iOS 17, and I can't narrow down the how and the when of it happening, but over the last week or so I've found that every couple of days Siri seems to lose their voice. By this I mean the high quality voice that's used when they speak seems to disappear. I notice this when I ask my phone or headphones a question or to do something, and I get a really low-quality voice that speaks back to me.
The voice itself seems to be a version of the voice I normally use, but like it's using an on-device much-cut-down version. If I go into the settings to check what voice is selected, it's the one I normally use, but it wants to download it again.
Sure enough, once the download is complete all is good again. I've not kept track of when it happens -- and I think I should from now on -- but it feels like it happens every couple of days; I almost always notice it first thing in the morning, the first time I ask the phone or the headphones something.
I hope it is some sort of iOS 17 weirdness and is gone when 17.1 turns up.