Secular just means anything not sacred. I don’t think the secular song “Happy Birthday” is a sinful song to sing. I also don’t think singing about the love someone has for their wife or daughter or tractor is bad. It’s bad when we start talking about how inappropriate this one girl in the club was and how it got us lusting after her in our heart. Or when we start talking about how much we want wealth and fame and everything for ourselves. Or how we are god of our own lives and no one can tell us what to do.
Music itself isn’t a moral concept. It’s amoral. It’s not good or bad. But like many other amoral things, (wealth, fame, etc) it can carry or amplify a moral message. Music is holy and unholy based off of the content of the lyrics.
Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things. Philippians 4:8
And probably the best passage for evaluating media:
Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God. But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control. Against such there is no law. Galatians 5