From what I recall, this wasn't mentioned at either WWDC 2023 OR the 2023 iPhone Announcement Keynote, but this is definitely one of my favourite things in iOS 17.
ALL NEW TONES!
It's been years (maybe over a decade?) since Apple last updated the default tones (ring and alert). They have long grown stale in my opinion, and were in dire need of an update to something more modern. Well, without any fanfare at all, Apple quietly updated the selection of default tones, and they are magnificent!
If you haven't already, I urge you to check them out and update all of your default sounds - especially your default ringtone and messages alert tones as those are the ones you likely hear most often. I think you will find them futuristic and modern, breathing new life into your alerts - they are respectfully crafted, and not gimmicky like some of the old ("classic") ones.