The best answer to the question is:
Her memories related to music are considered important to her, so her brain still reacts to this sound.
Research has shown that music can evoke memories and emotions, even in individuals with Alzheimer's disease. The neural pathways associated with music often remain intact longer than those associated with other types of memories. This may be due to the emotional and personal significance of music in a person's life, which helps facilitate recognition and recall despite memory impairments.