En bref
The demon-oppressed man’s muteness was indeed caused by the demons. The word describing him, kophos, could refer to either deafness, muteness, or both.
Yes, the Greek word here translated as mute, kophos, can either mean deaf, mute or both. We do know that this particular man was mute, for he could not speak (Matthew 9:33a), but he could have possibly been both deaf and mute.1 The two blind men had barely left when a demon-oppressed death-mute man was brought to Jesus.2 This man could not communicate his condition to Jesus but had others speak on his behalf. His muteness, however, was not a natural condition, but was caused by a demonic power.3
32 Comme ils s'en allaient, voici, on amena à Jésus un démoniaque muet.