Causes of mumbling include the speaker lacking confidence, being tired or sick, or not focusing on what he is saying. Nervousness is a common cause of mumbling in children and adults. There are also medical conditions, such as dysarthria, that cause mumbling,
A speaker may mumble when answering a question if he doesn't have a good answer. When the speaker is multitasking, mumbling occurs because a lack of focus negatively impacts speaking skills.
Dysarthria is a difficulty saying words due to nerve, brain or muscle issues. Nerve issues can be caused by diseases such as cerebral palsy, multiple sclerosis and Parkinson's disease. Muscle damage can result from head or neck trauma, or surgery in that area. Brain damage can result from a stroke, dementia or a traumatic brain injury. Dysarthria due to muscle damage and brain damage can improve. Dysarthria due to diseases that affect the nerves typically doesn't improve, and certain nerve diseases result in loss of speaking ability. Temporary dysarthria can be caused by alcohol intoxication or as a medication side effect.
Several mental illnesses, such as aphasia and catatonic schizophrenia, cause logorrhea. Logorrhea is a communication disorder that includes mumbling, particularly mumbling to oneself. Depending on the cause, logorrhea may improve with medication.