This is the weirdest issue I've ever encountered with my PC. Every so often, my sound will start playing back at a lower volume. This happens when watching video, listening to music, all independently.
It usually lasts anywhere up to a minute, after which it will turn up again.
The weird thing about it is that the volume control in Windows remains at 100%, even though the volume is audibly a lot lower. (No, I'm not going deaf, it's just my PC. I checked.)
I just have no idea where to go to troubleshoot this, even.
I'm using Windows 7 64-bit with an on-board Realtek sound-card.
Oh, just in case someone finds this question on Google or whatever, making sure this is on Do Nothing may fix your problem. Unfortunately, it did not work for me.
My settings all seem fine. This is my audio slider. (Took the screenshot while the issue showed.)