Any app that plays audio, YouTube, Netflix, YT Music, Chrome, whatever it might be, so it's definitely an OS wide issue as opposed to an app. It's the built in equaliser in Settings > Sound and vibration > Sound quality and effects > Equaliser.
Like I said when it happens it basically just disables it. I haven't actually checked what it says in the settings there when it happens, I'll do that next time it does though. Just turning the display off and back on then changing volume brings it all back.