I'm a deaf person with a Nexus 5X. I almost never want my phone emitting any kind of sound.
When I had a Samsung Galaxy S4, there was a configuration option to mute all sounds independently of the various volume settings.
Since then, Android has released their do-not-disturb mode. That doesn't really fit in with my usage because that ties in with the whole notification system. I want my phone to function as if it was not in the DND mode, just without sound.
I looked into key remapping solutions to prevent accidental buttonpresses (phone holder, pants, whatever), but that doesn't prevent applications from changing the volume. e.g. Google Duo assumes that all videophone calls users will want audio regardless of the audio settings prior to opening the app. That absolutely makes complete sense, so that's why I'm asking for a lower-level way to disable the speaker.
I did look at several muting-related questions, but those seem to be more of one-time actions - "I want to turn off sounds right now, but I'll adjust them later"
So, the question is, as the title states, how to selectively stop the speaker from functioning that cannot be overridden by any app. How would I proceed to go about doing that on a Nexus 5X?