Turn down the volume. I know dumb answer, but I found that the ringer volume controls my Dexcom alerts and the regular volume the volume for things like streaming. So, I need to turn down the ringer volume when I want to reduce the alarm sound.
We put all to vibrate but of course you can’t control the low alarm. Just try and catch it before it gets low. Set it at a good number before the urgent low sounds.
I'm assuming you're using your phone? As someone said, it's part of your phone's volume. All phones are different. Most have a volume panel, with different sliders that control the volume on different things on your phone. It should be the slider that controls the volume for all your apps. You might have to pay with them to figure out which one controls the volume on the Dexcom app.