How do I disable hidden volume changes on Windows 10?

I finally stumbled upon the core of the issue. The program affecting it was "Dolby Digital Plus Advanced Audio". To turn some of it off, I could find it in the Control Panel and turn off an effect called "Volume Leveler". But to completely remove it I later found out that I had to disable Dolby from affecting the playback device:

Thanks BramMooij for your helpful suggestions.

Try what is in the link, that might solve your issue if Windows incorrectly assesses which apps use communication.

This describes the same procedure, but in the link you will find pictures as well. Go to:
Control panel -> Hardware and Sound -> Sound. From there, go to the communications tab. There you will find the settings: "When Windows detects communication activity". The following options are available:

• Mute all other sounds
• Reduce the volume of other sounds by 80%
• Reduce the volume of other sounds by 30%
• Do Nothing

Select do nothing to possibly fix your problem.

http://www.tekrevue.com/tip/windows-reducing-system-volume/

Sorry if I'm mentioning other stuff that you already tried, but I cannot work out what exactly you did from your post (although I should have spotted that you had already tried my above answer). Here are some other options:

Some people have reported that this issue can be fixed by disabling front panel jack detection. Other people reported that reinstalling their keyboard, with the correct drivers, solved their issue. Some people had their mouse (especially logitech) causing this issue. Even someone had a faulty RAM module causing this issue.

Judging from the randomness of the solutions, I would definitely open the device manager and inspect if any devices are not properly installed (identified by exclamation marks in a yellow triangle). If this is not the case, I would try a different mouse and a different keyboard (first try one, then the other).

Source: http://answers.microsoft.com/en-us/windows/forum/windows_7-pictures/my-volume-slider-under-windows-7-keeps-going-down/2769853f-e412-4c4d-8b55-6885588a528f

Here is how I fixed it. In sound settings, you go to properties, in my case speakers. The Enhancements tab has some effects enabled by default, so I turned them off. And voila, no more sound ducking. hope that helps

just fixed my problem. Non of above solution worked for me. So what i did: opened Realtek HD Audio manager, clicked analog input (the first plug of the three on the left - matching my ports status) on the very bottom left and set that i plugged speakers not the headphones. But watch out - this settings could re-set -.-

In my case i don't have the tab "Enhancements"; instead, in the "Advanced tab" there is a checkbox that says "Enable Audio Improvements" inside a section titled "Signal improvements".

I disabled it and it behaves properly now.

Please note that my OS is in Spanish and I translated. I am not sure the same names are used in the English version.