many user still facing this issue. This issue was introduce in windows as a windows FEATURE.
To narrow down this issue, take out permission of all application accessing to microphone, then cross check one by one by enabling one permission at a time. This way you can find which application/services causing the problem.
For me whenever i open(or keep running in background) browser, it causes auto volume adjustment even though i disabled auto adjustment (SYS >SIUND>COMMUNICATION>DO_NOTHING) i hope you already did this.
Regarding permission: SYS>PRIVACY&SECURITY>MICROPHONE>MICROPHONE-ACCESS> first disable all then enable one by one.
If nothing works then manually select the driver (even if it warn un supported) STEPS mentioned below:
DEVICE-MANAGER> SOUND-VIDEO-GAME-CONTROLLER> REALTEK-AUDIO>DRIVER>UPDATE-DRIVER>BROWS-MY-COMPUTER>LET-ME-PICK> disable SHOW-COMPATIBLE> select MICROSOFT on left windows > and on right side chose HIGH-DEFINITION-AUDIO-DEVICE-VERSION latest one
at the end i can only say best of luck, since i know this issue was very serios to me since on daily basis i work on voice over recording.