I'm using a Nexus 5, it's using the stock ROM, it's unbranded, bought from the European Google Play store. I've only noticed this issue since upgrading to Marshmallow, but I didn't use the headphones much prior to that, so it may have been an issue then as well, I'm not sure.
Every few days, while I'm listening to music on my headphones, the phone reduces the volume down to around 30%. I have to unlock the phone, turn the volume up, then select okay when it comes up with a warning about turning the volume up.
It's got to be one of the most annoying "features" of the phone. I don't know why it does it, or how to override it. I read a post on the XDA forums where people said it was due to some European law, but there was some descent there.
I have a volume controller on my in-line cable, so I am more than capable of getting the volume right for myself. Can anyone tell me if it's possible to override this, so the volume setting stays at the level I dictate, not the level the phone thinks is safe?
I'm aware I can lock the volume with "Persist" or similar but I would rather have control, as sometimes I want to adjust the volume depending on song, background noise and what I'm focused on.