The phenomenon you're referring to is known as the "McGurk effect," where visual information can influence our perception of sound. It occurs because our brain integrates information from multiple sensory modalities, such as vision and hearing, to form a coherent perception of the world.
When we see someone speaking, our brain processes both the visual information of their mouth movements and the auditory information of the corresponding speech sounds. In normal circumstances, the brain combines these two sets of information to perceive a unified speech perception.
However, under certain conditions, when there is a mismatch between the visual and auditory information, our brain can be tricked into perceiving a sound that is different from the actual auditory stimulus. For example, if the visual information suggests that the person is saying "ga," but the auditory information is actually "ba," we may perceive the sound as a combination of the two, such as "da" or "tha."
The McGurk effect highlights the complex interaction between our visual and auditory systems. Our brain integrates these sensory inputs to construct a unified perception, and when there is a discrepancy, the brain may favor one modality over the other or create a blended percept.
The reason why we may hear sound after seeing light is because the brain is processing both visual and auditory information simultaneously and integrating them into a coherent experience. This integration can sometimes result in the perception of a sound that is influenced by the visual stimulus that preceded it.