Sounds like she is loosing her hearing. Try a dog whistle and see if she still hears that pitch.
Well she's not completely deaf. I don't have a dog whistle but I went up behind her outside and called her. Nothing. When I clapped my hands though she turned her head. She kept stopping until I turned toward her and then came in the house. I took a pot and droped it a few feet from her when she wasn't looking. She turned her head but didn't jump. My husband has a deep voice and he called her from another room. Her ears never moved.