Have you tried starting up in "Safe Mode" and see if the volume level changes when it starts in there? While you can't access most programs in there, it will tell you if it is something on the computer that is the cause. If it doesn't happen in "Safe Mode" then it is a program, virus, malware, etc.
How to enter "Safe Mode" when booting the computer.
In Windows 8 and 10...
As your computer restarts, press F8 (possibly a few times) to enter "Safe Mode"
a. Press the "F4" key to Enable "Safe Mode".
(The computer will then start in "Safe Mode" with a minimal set of drivers and services.)
b. Press the "F5" key to Enable "Safe Mode" with Networking.
( Once "Safe Mode" with Networking starts, Windows is in Safe Mode, with additional...