Galaxy S6 :: Alarm Sometimes Makes No Sound?

I have an AT&T Galaxy S6 Edge on 5.1.1.

A couple of weeks ago I slept through my alarm, at least that's what I thought had happened. When I woke up some time later, there was a notification that I had missed an alarm. It seemed strange, since I've never not been woken by the sound of an alarm before.

The alarm worked fine for the six months before that, and has been working fine for the last few weeks.

Today I was awake before my alarm went off. I then watched my phone screen light up with the alarm, but no sound. I turned off the alarm and waited for my backup alarm. Same thing - no sound.

I did not have Do Not Disturb mode turned on. I almost always keep my phone on silent, and the alarm has always worked. I'm using the default Samsung clock/alarm app.

If it's as simple as using a different alarm app, that's an easy fix, although it doesn't leave me with much faith in Samsung. My concern is that it's a system issue. Unfortunately since it seems to be a sporadic issue, testing the alarm isn't going to tell me definitively if I've been able to fix the issue. Unless I can identify exactly what is causing the problem, I'm going to have to buy a dedicated alarm clock because I can't risk missing an alarm again.