Once again, let the title be your guide. The song "Go Tell It On the Mountain" is the source of the novel's name, and you, like Dolly Parton, might know it best as a Christmas song. And from that very first hymn, music continues to be an important part of the narrative in Go Tell It On the Mountain.
Music is the basis of the worship services in the church. It brings on what's called "the Shout," when someone in the congregation is overcome by the spirit and lets it take over his whole body:
Someone moved a chair a little to give them room, the rhythm paused, the singing stopped, only the pounding feet and the clapping hands were heard; then another cry, another dancer; then the tambourines began again, and the voices rose again, and the music swept on again, like fire, or flood, or judgment. (1.1.17)
The music is like the force of God; it's compared to "fire, flood," both forms of God's judgment in the Old Testament. The music itself is seen as God judging the people. If the music moves you to shout, it shows that you are holy; if not, well, that's a form of judgment, too.
All of the scenes that take place in the church are punctuated with italicized lyrics from the hymns that the congregation is singing. It's like a reminder to the reader that this is a multimedia moment, not just words on the page. It's a way of bringing the scene to life. Check it out:
Elisha began a song: "This may be my last time," and they began to sing:
"This may be the last time I pray with you,
This may be my last time, I don't know."
As they sang they clapped their hands and John saw that Sister McCandless looked about her for a tambourine. (1.1.288-89)
Sister McCandless looking around for her tambourine shows that music is a very important part of the religious ceremony that's about to take place. In fact, it's so important that it's a requirement. John watches them, "singing with them—because otherwise they would force him to sing—and trying not to hear the words that he forced outward from his throat" (1.1.290). For John, singing is a way of participating in the community, of hiding his anger and rebelliousness.
While the hymns punctuate the saintly church service, another kind of music comes up a lot in the novel as well: the blues. But blues is seen as raunchy, crude, and provocative. This "sinful" kind of music is lumped together with all sorts of ugly stuff in John's mind:
John heard them in the bedroom behind him, over the sound of rats' feet, and rat screams, and the music and cursing from the harlot's house downstairs. (1.1.6)
Rats, rat screams, cursing, and… music? We would think that one of these things is not like the others, but John sees the blues as the soundtrack of sin and squalor.
That association continues throughout the novel. Often when a character goes into a sinful place, the blues shows up again. When we're to understand that Frank's a good-for-nothing husband, it's as simple as saying, "Frank sang the blues, and he drank too much" (2.1.73), for us to get the gist: he's a dirtbag.
This music gets a definition when Gabriel goes out into the field to preach, running from his own sin:
The music that filled any town or city he entered was not the music of the saints but another music, infernal, which glorified lust and held righteousness up to scorn. (2.2.287)
Blues and jazz are seen as having the power of causing people to sin, just as church music is seen as having the power to bring about salvation.
When Gabriel meets John for the first time he tries to reconcile the two types of music. When John is just a bitty baby, he is attracted to the blues music that he hears in a nearby apartment. His mother explains, "Johnny want to hear some more of that music. He like to start dancing when he was coming up the stairs." (2.3.217). He's just an innocent baby, so surely he's not a sinner for liking the blues?
Gabriel's response is actually (for Gabriel) really freaking sweet. He tells the baby:
"Got a man in the Bible, son, who liked music, too. He used to play on his harp before the king, and he got to dancing one day before the Lord. You reckon you going to dance for the Lord one of these days?" (2.3.218)
This is one of the rare moments in the novel where Gabriel's kindness and humanity shine through. With this statement he suggests that all music has the potential for holiness, and, by extension, all people have the potential for holy action. It's moments of kindness like this that make it so difficult to see Gabriel's heart harden towards John. When John finally does dance for the Lord, on the day of his conversion, Gabriel sees it as a threat.