Suppse that I had a device that would cause a mild but sensible vibration in the nasal membranes of the person I pointed it at. Absent consent or a significant reason, it would be wrong to use this device on a stranger.
But the same is not true if we replace nasal membranes with the tympanic membrane: we routinely vibrate the tympanic membranes of strangers with neither consent nor significant reason, say when we ask a stranger on the street for directions.
In both cases one is inducing a physical change of arrangement of body parts in the other person without their consent. We may suppose that hedonically there is no difference: perhaps the vibration and the speech are both mildly unpleasant. The case can be tweaked so that the impact on autonomy is greater in either case (e.g., the unwilling listener may identify themselves as the sort of person who doesn’t listen to arguments) or so that it is equal.
It is tempting to say that we have a default consent to hearing others out. But default consents can be withdrawn, and we are permitted to vibrate tympanic membranes even against the express directions of their possessor. If during an argument someone says “I don’t want to hear another word!” it is not morally wrong to respond verbally nonetheless.
This implies that the need for consent does not supervene on hedonic or autonomy facts. It depends on details of the intervention that go beyond these.
The fact that in my thought experiment an apparatus is used in the nasal but not the aural case is not relevant. If one speaks through a speech generating device, as famously Hawking did, one is no less permitted to vibrate strangers’ tympanic membranes with the speech. And it would be just as wrong to go up to strangers and blow air into their nostrils in order to vibrate their nasal membranes as to use a device.
So what is the difference?
The difference, I think, is that it is a part of the proper function of the tympanic membrane to receive speech from random strangers, whether one consents to this or not, while the nasal membranes have no such proper function. It is as if our human nature gives permission to others to speak to us, but does not give such a permission for nasal membrane vibration.
I think this is difficult to account for in anything other than natural law or divine command ethics.