^Yes, obviously its real-life purpose is as a visual cue. But I was trying to think of how the ear might be activated, and it occurred to me that said visual cue could be repurposed to serve an actual in-story function.
The thing is, although it's a given that Rudy Wells was decades ahead of the state of the art of bionics, it's a little hard to believe that, working in the 1970s, he could've devised such a sophisticated brain-bionic interface that the implant would be able to respond to volition alone rather than some physiological trigger. With Steve's eye, the magnification isn't entirely dissimilar to what the eye does naturally when shifting focus; perhaps the bionic eye automatically zooms when it detects that it's being focused on something at a distance (although I confess I'm not sure how a single bionic eye could have that kind of depth perception). Or maybe if he squints in a certain way, it activates a pressure sensor. And the infrared vision could be activated by the same nerves that cause the pupils to dilate in dim light, say. So I can buy that there's a physiological trigger to those functions because they're analogous to things the human eye can already do, things it has nerves and muscles to control.
But there's nothing intrinsic to human hearing that would have the capacity to turn up the gain. "By thought alone" doesn't really work, because even our brains wouldn't be wired with a "turn up the volume" trigger, because that's not something we can do at all.
Although I suppose it is possible that it could be like the more recent experiments into controlling devices through brain telemetry -- the devices don't actually read your thoughts, but you can train yourself to concentrate on a particular thought and create a state of brain activity that the device is programmed to recognize as a trigger. So yes, I suppose it's possible that she could've been trained to adjust its gain through some kind of neural feedback, but the idea that there was a proximity trigger in the bionic hand is an interesting possibility as well.
And yes, I'm well aware that the bionics couldn't really have worked as shown. Steve and Jaime would've needed heavily reinforced spines and hips to be able to achieve the feats of strength shown, and half the time their left arms would've been totally incapable to provide the bracing or leverage they were shown using them for. Not to mention, when they're running at 60 MPH, how could they be pumping both arms superfast to keep pace with the legs? So, sure, there's a ton that doesn't really work, but it's fun to speculate about how it could work.