I found a solution for the same problem. I hope it helps all of you.
The key arises from yet another classic "Feature" being described unintuitively in the Settings.
On Android 10, the Alarm app has a Settings option. The first item in the General subsection reads "Vibrate for alarms and timers".
I suspect this is ON for all of you having the problem.
Intuitively, people read this as "yes, I would like all my alarms and timers to vibrate, thank you!".
But a close reading of the second sentence of the tiny text below makes it clear that this option makes it so that alarms and timers ONLY vibrate and do not play sounds when on Mute or Vibrate. So, in fact, a useful setting, but it operates the opposite of what one first thinks it does.
So, you must DISABLE this (aka Turn if OFF) to let your alarms and timers still play sounds when your phone is otherwise muting or vibrating everything else.
I really do hope this helps everyone who upgraded from a much older build to a much newer build and is still navigating its new quirks.