This is NOT solved. I clicked on the box because I thought it was a dropdown box, and it erased my reply.
I have not been able to find any actions that I do to correlate with the volume turning itself down. I do not use "do not disturb", but I DO use the ring silence button....but I tried that, and it did not change it. I also do not use headphones, or different ringtones. I did download a ringtone app, but I could not get it to work, and have not used it. I will smoke it from my phone.
I use Bluetooth often....driving and at work. It will be difficult, but I will try turning it off when I can to see what happens.
It does this both at night, and during the day. I charge my phone only at night.
The phone has not been in "low power" mode, and I do not use Facebook on my phone (two other things that friends had mentioned).
This happens 2 to 3 times per week. This is important to me because I am on call at night, and I need to hear the phone to wake up if I get an emergency call.
Thanks for your help!