This is the wrong question. (It’s like the unmarried couple who asks “How far can I go?”) It's tempting to draft up a list of the acceptable music versus the taboo, because it relieves us of having to think through this biblically and humbly. Lists of rules can also promote an arrogant sense of self righteousness with people walking around feeling holy because not a single MP3 in their car fits under the category "secular." It’s much easier to hide behind the "Here's what's acceptable" rule than it is to aggressively pursue God's glory through responsible music selection.

"If you have died with Christ to the elementary principles of the world, why, as if you were living in the world, do you submit yourself to decrees, such as, 'Do not handle, do not taste, do not touch!'...These are matter which have, to be sure, the appearance of wisdom in self-made religion and self-abasmenet and severe treatment of the body, but are of no value against fleshly indulgence" (Col 2:21; Col 2:23). 

Instead we should ask this: How can I exalt and delight in Christ through the music I listen to? That's a far better question. Instead of trying to find out what we can get away with, it focuses on how we can please God. 

This also helps distance us from the temptation to a legalistic approach that bans anything with screeching electric guitars or pounding drums, or that doesn't directly talk about God. Frankly, to try pinpoint a certain style of music as sinful is to go way farther than Scripture allows. The heart motive of all music enjoyment should be to enjoy the One who gave us the gift of music. Styles in and of themselves are not sinful or holy. 

I can worship my Savior while listening to a Beethoven classic or while tap dancing to a 1930's rag tag with a pinch of Big Band, even if the songs themselves say nothing directly about God. Yet they indirectly tell me much about God, for they are part of His creation (Rom 1:20). Music itself is an expression of the wonders and excellence of God, and it is sin that has twisted it. Instead of trying to draw a line between "bad music" and "good music," see music as God sees it: a marvel of His creation, yet often twisted and tainted by sin. "For everything created by God is good, and nothing is to be rejected if it is received with gratitude; for it is sanctified by means of the word of God and prayer" (1 Tim 4:4-5). Tim Keller wrote, “Everything in the world is properly understood only if we see 1) it was created good, 2) it is fallen and marred by sin, 3) it will be redeemed in Christ.”

If the focus is to praise His holy name, we won't waste time trying to decide which styles of music have a place on God's MP3 rack. Instead, we will ask, how can I please my Savior this day, whether that be through a rock 'n roll classic, heart-pumpin rap, or Charles Wesley's Oh For a Thousand Tongues to Sing. 

Admittedly (I hope this is obvious) any music that praises sin by using filthy language, suggestions of murder, suicide, or fornication or music that promotes "the world revolves around me" mindset is sinful and a Christian cannot listen to this kind of music and honor Philippians 4:8 at the same time. 

However, to say that unless the music directly talks about God it's sinful or shouldn't be listened to is foolish. Does the person advocating this position ever say anything that does not directly praise God? “Could you pass the beans—oops! Praise Jesus! Could you pass the beans made by Jesus so I can feed my body so I can praise Jesus some more today?”

But my intention is not to blur the distinction between Christian and secular music. When someone says "Christian music" today he is talking more about a genre (kind) of music style than music that is directly Christian. Many songs that fall into the Christian genre are not Christian at all. A large selection of Christian songs use the name of Christ but say things about Him that are not true or encourage a response to Him that is not biblical.

However, using the term "Christian music" defined as music that directly and biblically exalts Christ, here is the main distinction I see between Christian music and secular: Christian music is very close to special revelation, secular music (that does not promote sin) is more like general revelation. 

Christian music speaks directly to God's character (like the Bible), non-sinful secular music indirectly points to God's character (like creation). 

Augustine summed it up nicely: “Love God and do as you please.” Yea, it sounds horrible at first. But think about it. This is the essence of living for God! If you love God, what pleases Him will become what pleases you.