Hi CC1967,
welcome to the Apple (user-to-user) Community!
Re: How to fix no alerts or vibrations for sound recognition on iPhone 12 ?
Wondering if you checked whether Sound Recognition is on or off in Control Center:
Eg: Apple Support's article: Recognize sounds using iPhone - Apple Support
says: Tip: To quickly turn Sound Recognition on or off, use Control Center.
Also: for custom sounds you may have added:
You may want to check you have tapped "Start Listening", and followed the onscreen instructions.
(See the bottom section in the above article.)
If needed: You may want to try updating to the newest version of iOS 17
(NB: It is always wise to back-up your data before updating: Back up iPhone - Apple Support)
You can check if an update is waiting to install in Settings > General > Software updates.
(About iOS 17 Updates - Apple Support says iOS 17.5.1 is the latestversion)
If you need more help with using this feature:
You may want to contact Apple's Accessibility Support Team:
How to contact Apple for accessibility support - Apple Support
or
From Apple's Accessibility Support Website
(NB: You can "Get Support" (online chat / phone / email etc.) from the link at the bottom of their website:)
Accessibility Support - Official Apple Support
All the best :-)