what are some of the causes of spach disorders (such as mute)?

Answer 1

Here are some speech disorders and their causes:
apraxia - caused by damage to parts of the brain
aphasia - caused by damage to parts of the brain
dysarthria - usually caused by a stroke but can be a brain injury
oral cancer - self explanatory
stuttering - exact cause unknown

These sites will give you more detailed information:
http://www.nlm.nih.gov/medlineplus/speechandcommunicationdisorders.html
http://www.kidshealth.org/teen/diseases_conditions/sight/speech_disorders.html
http://en.wikipedia.org/wiki/Speech_disorder
http://www.asha.org/public/speech/disorders/