Psalm 40:3 And he hath put a new song in my mouth, even praise unto our God: many shall see it, and fear, and shall trust in the Lord.
God, the Holy Spirit, has worked faith in Jesus into our heart, which produces the result of a new song and praise to God with our mouth. By speaking out loud about how good God is many will see it and believe the Lord. The new song in the heart comes out through the mouth. People will notice and come to fear and trust in the Lord.
The Christ in us sings a new song of praise, singing psalms, hymns, spiritual songs, and making melody in our heart to the Lord. The New Man is continually rejoicing in the Lord. He goes through the day singing a new song. The Gospel message is always with us and for us, and so the rejoicing response is always present. Praise is a way of life.