When you make a mmmmm sound while sleeping and this sound is very long, you need to consider the following conditions: The first condition may be the sound of snoring. If there is a disease of the nasal cavity as well as a disease of the throat that may cause this condition, you can go to the ENT department for an examination to clarify the presence of this disease. The second condition may be a form of seizure. If there are twitching of the limbs and such sounds are present, epilepsy should be considered. You can go to the neurology department of the hospital and have an EEG and, if necessary, a video EEG done to clarify whether the condition is caused by epilepsy. In the third case, it may be a sleep disorder, the nerves are more excited, and this kind of sound is caused by dreaming, this situation can be improved by taking some oral drugs to regulate nerves and help sleep.