If it were me, I'd probably make the vision monochrome, and as though he were seeing through a haze. An example of what I'm thinking would be whenever Frodo put on the ring and the world around him became less distinct, but the things tied to the ring were revealed for what they were. I'm not sure what your reasoning was for tying it to his hearing, but to me that doesn't really make sense. Why would the magic decide to attach itself to his ears instead of skin? Or why not replace his eyes? 

This is just a suggestion, but perhaps you could simply have it be a sixth sense that doesn't rely on any other sense, but is blocked by solid objects (such as walls). According to the premise of the RP, magic exists in everything, so a solid wall would effectively be a barrier of magic for this vision. This would mean that a spell would have to be immensely powerful for him to see it through a solid object, and would also make the vision less distinct if looking through water or at a distance (due to the air itself being permeated with magic as well). He'd still have 360 degree sight, and if the spell is powerful enough the ability to sense it through a wall, but it doesn't have the added complexity of tying it to one of his original five senses. Essentially, zero magic would be black, pure magic would be white, and so the world around him would be varying combinations of the two. He'd be able to see almost perfectly in his immediate proximity, but the farther he looks the blurrier things get (which would also give a reason for why he uses a giant sword and short range throwing knives and has no bow).

Put even more simply, it's a sixth sense that lets him see magic but is impeded by the very thing it perceives. Simple enough for a single sentence, and easy to extrapolate from regarding strengths and weaknesses.

That's my two cents on the issue.