No, hearing a loud sound during sleep would not make you permanently deaf. While it is true that the forebrain, which is responsible for processing sensory information, is less active during sleep, the auditory system is still partially active. The brain is capable of filtering and processing sensory stimuli during sleep, including sounds, although the level of awareness and responsiveness is reduced compared to wakefulness.
If a very loud sound occurs during sleep, it can still be detected and processed to some extent by the auditory system. The brain may trigger a partial awakening or a shift to lighter stages of sleep in response to the loud noise. This can result in brief moments of wakefulness or a shift in sleep patterns. However, once the sound subsides, and if no damage has occurred to the auditory system, normal sleep is likely to resume.
It's worth noting that exposure to extremely loud sounds over an extended period, whether during sleep or wakefulness, can potentially cause hearing damage or lead to temporary or permanent hearing loss. It's important to protect your hearing and avoid prolonged exposure to loud noises whenever possible.