How ALS affects communication.....

First of all, because ALS is a disease that affects the communication between a person's brain and motor neurons there is a loss of function in muscle movements. Speech is a combination of muscle contractions, respiration, and coordination, all of which are affected by ALS.
In the beginning speech may become slurred or 'thick' and it will be difficult for a person to project their voice. These are all initial symptoms of ALS for many people. Also, because respiration is affected a ventilator will be necessary for life support, this too affects speech. Overtime, as the disease progresses an overall paralysis will overtake the person's body, this includes the mechanisms used for communication, making speech impossible.
Because their is no cure or any real treatment for this disease the only thing to be done is to make the person as comfortable as possible.