Can someone knowledgeable say why drunkeness would.cause inability to speak while all other bodily functions work?

Prolonged heavy drinking, or prolonged binge drinking, can result in permanent damage to the brain and nervous system. Too, there is a disorder called Selective Mutism. Selective mutism is a severe anxiety disorder where a person is unable to speak in certain social situations, such as with classmates at school or to relatives they do not see very often. A child or adult with selective mutism does not refuse or choose not to speak at certain times, they're literally unable to speak.