You could be experiencing audio clipping..though generally this will be noticeable when something 'large' happens on screen (explosion when watching a movie) or high percussion during music.
You will hear clipping (distortion) mostly when amplifier is pushed too much for a certain frequency. You could try reducing gain across all frequencies to see if that helps.
Equalizer APO (with Peace Overlay) has an anti clipping option that tries to correct the audio stream before clipping occurs. You could download that and test it out. You will most likely need to remove armory crate as armory crate tends to hold onto its settings be default, so any changes in Equalizer APO wont affect audio when Armory is installed (from my experience anyways)
https://sourceforge.net/projects/equalizerapo/I could be totally off the ball, but from what Im reading, this sounds whats happening. (sounds like a far easier option to fix a software issue, then having to send off your headphones etc).
Good luck
😉~BS~