How can I go back to old Siri voices?

iOS 18, with Apple Intelligence, brought a new Siri manner of speech. Personally, to me, it's much more natural sounding. But yes, it's different and with all things new, eventually, we get used to new things and forget how things were before the change. I'd personally be disappointed if Apple made Siri sound as it did before. But you can certainly use this link to give feedback to Apple --> Product Feedback - Apple

I'd encourage you to give it a chance and chances are you'll get accustomed to the new Siri voice (it doesn't really matter which accent you choose) and maybe, just maybe, even appreciate it. One thing we count on with technology is change.

in settings , Siri. you can edit the voice. There is now a larger variety of multi sex voices