The timer in the Google clock app on my new S21 only makes a sound if the screen is unlocked. This makes it pretty useless. I use the timer a lot and I've never had this problem with previous Android devices.
(The alarm in the same app, which I use every morning, works perfectly)
Apologies. This isn't a Samsung problem. I've just been reading reviews of the Google Clock app in the Play Store. Apparently this is a bug in the latest version of the app in Android 12. It's all Google's fault.