Runie has really bad hearing.
That's pretty much it, she just doesn't hear things. It affects her gaming and it also affects her singing, she always has trouble with timing music to synchronize, and her ability to hear and match rhythms isn't great either. I think there's a part of it that has to do with how she focuses her attention, like, she can't read game interfaces like heath bars, or popup text, or "press E to ____" either, these things just don't register to her unless she focuses directly at them or they're right on top of something she was already looking at. Something similar goes on with her ability to hear audio its just not as easy to recognize.