The AIM field is always there. However, it most likely takes the form of slight pressure - and sound waves themselves are pressure waves. It sounds like it may affect the sound a little, just not that much, and much less if Akito's very close or, hell, even hugging him/got his ear up against him. I can go with whatever makes the most sense for "animu physics" that you're comfortable with. The brain damage itself is in the frontal lobe, and affects his speech/language, motor, and calculation skills. The device around his neck has the network terminal as described in the post, though I don't know much about the inner workings. Just that there is a transformer that allows for his brainwaves to link with the connection to the Misaka Network (lots and lots of electromagnetic brain waves) - which he has a constant connection to. You also have my permission for Akito to pick up on its presence if you think his hearing is keen enough for that.

Let me know if you have any other questions/comments.