First, I really appreciate Chris Marr's question (below). Yes, the mouth speaks out of the heart, whether wicked or good (Matt. 12:34). That is why it is so important to open our mouths to cry out for salvation and to praise our Lord and Savior - "Hallelujah, O Lord Jesus!" There is salvation in his name and only in his name (Acts 4:12), and we know that "no one can say, Jesus is Lord! except in the Holy Spirit" (1 Cor. 12:3). Since the beginning, God's redeemed people have been characterized as those who call on the name of the Lord (Gen. 4:26, 1 Cor. 1:2, Rev. 22:20).
Of all the short spiritual songs, this is my favorite. Singing this song has never failed to rescue me from a darkened situation. The enemy wants us to remain silent. Let us use our mouths to call (audibly) upon the name of the Lord.
Jeremiah 17:9 says "The heart is deceitful above all things, and desperately sick; who can understand it?" How do you know that it is the spirit and not the heart that is connected to the mouth?