Final answer:
A crossover of visual and auditory signals in the brain would likely involve damage to the optic chiasm or thalamic relays, as these structures are critical in routing sensory input to their respective processing areas in the brain.
Step-by-step explanation:
If signals from the eyes were being sent to the area of the brain responsible for processing sound, and signals from the ears to the area that processes vision, it suggests there is a disruption in the neural pathways that communicate sensory information to the brain. The optic nerves, which carry visual information, usually terminate at the lateral geniculate nucleus in the thalamus and then proceed to the visual cortex in the occipital lobe. Meanwhile, auditory information is processed by the auditory cortex located in the temporal lobe. A form of brain damage which would cause such a crossover of signals could possibly be entangled with the optic chiasm or the thalamic relays due to their specific roles in the routing of sensory input.
It is here in the optic chiasm where the partial crossing of optic fibers occurs, directing the correct half of the visual field to the appropriate hemisphere of the brain. Importantly, retinal processing is completed before visual signals reach the brain, involving bipolar cells and ganglion cells. Any damage leading to misrouted signals would have to occur after this intricate retinal processing, beyond the eye but prior to or within the brain's sensory processing regions.