What is the disorder where people can sing but not talk?

Aphasia.

Aphasia is a language disorder that affects a person's ability to understand or produce language, including speech, writing, and gestures. It can affect any part of language processing, including comprehension, word retrieval, grammar, and syntax. Aphasia is usually caused by damage to the brain, often from a stroke, but it can also be caused by other conditions such as brain tumors or head injuries.

People with aphasia may have difficulty understanding what others are saying, or they may produce language that is difficult for others to understand. They may also have difficulty reading or writing. The symptoms of aphasia can vary depending on the location and extent of the brain damage.

Aphasia can be a very frustrating and debilitating disorder, but there are a number of treatments that can help people with aphasia improve their language skills. Treatment options may include speech therapy, occupational therapy, and physical therapy.