‘Then was brought to him one who was possessed with a demon, blind and dumb, and he healed him, in so much that the dumb man spoke and saw.'
Matthew now introduces an example of someone who needs the power of the Spirit of God exercised on his behalf. He is possessed by a demon which makes him both blind, and deaf and dumb. The word used for ‘dumb' regularly includes deafness. It is no coincidence that these are the spiritual problems of Israel (Matthew 13:14-15), and Matthew has that in mind. The people of Israel are blind and cannot recognise Who Jesus is, they are deaf and dumb and do not testify to His Name. But once Jesus had healed the man he both spoke and saw. (Notice the minor chiasmus - ‘blind - dumb - spoke - saw'). So it could be for Israel if they would only look to Him. They would be able to ‘see God' (Matthew 5:8) and be able to testify of Him (Matthew 10:32).
Note that the man is described as ‘healed' which is unusual for the casting out of demons (but compare Matthew 15:28). It may well be that Jesus wants to connect it with the overall ‘healing' of Israel in Matthew 12:15. Normally a demon is spoken of as being cast out by a word (Jesus is never said to lay hands on a demon-possessed person).