Before the last upgrade on my S10 I was able to have the notifications/texts/phone on silent but still have my alarm have sound. Now, if I silence the phone it also makes the alarm silent.
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The first remedial step I frequently recommend is to do a hard reset using the volume down and power buttons. Just hold them down until the phone reboots, then check it after the phone settles down after the reboot, see if that clears the problem. This is usually a system glitch, I just tested it on my Note 9, recently updated, and with the phone ringer in the off position, my alarm sounds off fine.
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