An idol is, well, an idol.

Surely he cuts cedars for himself, and takes a cypress or an oak and raises it for himself among the trees of the forest. He plants a fir, and the rain makes it grow. Then it becomes something for a man to burn, so he takes one of them and warms himself; he also makes a fire to bake bread. He also makes a god and worships it; he makes it a graven image and falls down before it. Half of it he burns in the fire; over this half he eats meat as he roasts a roast and is satisfied. He also warms himself and says, "Aha! I am warm, I have seen the fire." But the rest of it he makes into a god, his graven image. He falls down before it and worships; he also prays to it and says, "Deliver me, for you are my god." They do not know, nor do they understand, for He has smeared over their eyes so that they cannot see and their hearts so that they cannot comprehend. No one recalls, nor is there knowledge or understanding to say, "I have burned half of it in the fire and also have baked bread over its coals. I roast meat and eat it. Then I make the rest of it into an abomination, I fall down before a block of wood!" He feeds on ashes; a deceived heart has turned him aside. And he cannot deliver himself, nor say, "Is there not a lie in my right hand?"
(Isaiah 44:14-20 NASB)

Over in Frank's meta, I made mention of a certain Mr. Schoenberg, who was a composer in the previous century. His purpose was to make mincemeat of the tonal system, so that all 12 notes in the scale carried equal weight; none would stand as a tonal center and there would be no possibility of resolution. Schoenberg had faith in the rationality of man, and wished music to reflect his hope in this mathematical rationality. This is the clearest example I can give of music being idolatry.

Now, it doesn't take long before someone says, "How can you say that Schoenberg is objectively bad or harmful?" I want to note here that I have chosen my words carefully. That's why I have not said that Schoenberg's music is bad, nor that it is harmful to listen to.

Idols are always crafted by someone, and a good idolater crafts his idol well. One can see the art of the idol. If the idol had not been well-crafted, the idolater probably wouldn't worship it. But it takes a worshiper of the true God to see the truth behind the false god, even while appreciating the craft that went into making the false god. But a true worshiper is not going to make the same kind of art as the idolater, because he is not in the service of a false god.