The analogy has two parts.
The text of Scripture is analogous to the notes on the musical score. They're the content. The right notes. They're already there, and anyone (who can read) can read them.
Preaching, in this analogy, is like the dynamic markers on the sheet. The time signature, the little marks (<) denoting a crescendo, the style and tempo headings (allegro, presto, con motto), the coda sign. They don't tell you what to play, but how to play it. (When is something repeated? Is this bit quiet in order to be solemn, or should I be playing these notes with joy? Where is the grand finale that I'll never forget?) They help you understand the movement of the piece, the effect it intends to have on its hearers and itself provides much of that impact.
A part of the value of this analogy, for me, is that it mirrors the reality that it is the Word of God that changes hearts. Preaching convinces, cajoles and/or exhorts the hearer to be moved by the text, much as dynamics grip, sooth and/or assault the senses in order bring the hearer into the experience of the music.
One of my ideals for preaching is that its purpose is to be an exposition. It ought to expose the truth of the text to the hearts of the congregation, in all its colour, strength, rebuke, gentleness, beauty, healing, grace and power. Put another way, perhaps more speculatively, just as dynamics allow the listener to hear the significance of each note, a sermon gets across the significance of each part of God's Word to the hearts of His people.
/end creative flow