I also had a similar issue. I had the iPhone 6 and had this issue, then switched to the iPhone 11 in September and had the issue again. I was really hoping going to the iPhone 11 would fix this, but it looks like the only thing that fixes is the battery life..
The alarm shows on the home screen and has the option to snooze or stop, but the sound does not go off. I use a song from my iTunes for my alarm. Everything was set properly, the same as the post before mine (mute was not active, not in do-not-disturb) the alarm setting was correct, the sound was on and loud enough per the Sound and Haptics setting.
I changed the alarm time up a couple minutes twice and the sound was still not working each try. I went into the alarm settings and tried selecting the alarm sound being used and nothing happened, when I selected another song available on this main screen it then started playing the new tune, when I selected my original song again it finally started playing and now the alarm works again.
This has only happened to me twice so I can't recreate the issue, but it is really scary to put my waking up in time for work in the hands of the iPhone anymore, looks like I'll have to by a cheap alarm clock...