He couldn't hear a word that they said. How can a person like that be converted? How could he respond to the center of Israel's life, the Shema (Deuteronomy 6:1-9) that begins "Hear, O Israel"?
He couldn't speak well. How could he keep the instruction to "Keep these words...Recite them to your children and talk about them when you are at home and when you are away..."?
This gentile living in a gentile world, not asking for help himself, not professing any faith in the ability of Jesus to help is suddenly able to hear--and to respond.
Mark is telling this story to people who would have known Isaiah's prophecy for the exiles of his time, "The ears of the deaf shall be unstopped....and the tongue of the speechless sing for joy." (excerpted from Isaiah 35:5-6).
We can read Mark's gospel as affirming that the exile is over, that Jews can return to a place that has been promised, and that Gentiles can join them.
(I was helped in this by reading Allen & Williamson's commentary on this passage in their Preaching the Gospels.)