As soon as I start playing a sound, the volume slowly goes down to about 5%. When I toggle "Mute" the sound comes back on for about 1 second and goes down again. I have tried all the solutions that I could find on the web, but no luck yet.
I have tried all 7 options as outlined here: Fix Volume Automatically Goes Up / Down in Windows 10
The next track on my playlist starts with full volume, but again, drops after 1 second. This also applies to the sound on e.g. YouTube videos. Any help would be greatly appreciated. :)
This issue is NOT happening when playing over Bluetooth or using my headphone.

Windows 10 ver. 2004.19041.329
Realtec Audio drivers (new)
New HP Pavilion Laptop

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Problem solved: Another day of terror but I fixed it :) This time I uninstalled the audio driver (as I have done before) but, I ticked the box "Delete also the driver files from your drive". After that the incorrect drivers were not automatically re-installed. Then I went to the Hewlett-Packard web site and had my laptop's model and serial number checked online. From there I downloaded the correct drivers and that did the job. The mistakes I made before were: downloading the drivers from the Realtek web site, and, updating the driver from Windows' Device manager. I suppose it all started after some Windows updates or automatic driver updates from Iobit Driver Booster that updated a wrong driver.