Following a discussion about the absence of musical restrictions in scripture, I heard perhaps the best (and actually only good) argument I have heard in favour of these restrictions. According to one young man, when David played a harp for Saul, evil spirits would leave him. In his words, this shows "we know that music without lyrics have a spiritual effect."
The question is, does it really? The first thing to note about the verse in question is that David played the harp when a spirit from God was upon Saul. In other words, there are at least two factors at play here, one of which I'm sure we'll all agree is significantly more powerful than music. A spirit from God was already upon Saul. If I was to guess, I'd say the distressing spirit was already feeling a little uncomfortable at this point. No brainer here: God > Satan.
But let's suppose the harp really was a contributing factor. Is it not possible that the harp resonates with Saul's spirit in particular? It cannot be denied that musical interests vary between people. Various things can also alter how music makes people feel, not least of which is associated memories. If you have good memories that are brought on by a specific sound, it doesn't matter what that sound does to other people, it's going to bring on the same emotion the memory itself does. The opposite can happen, too. If you associate a normally calming sound with a negative memory, it's not going to help you, it's more likely to harm you.
To illustrate this, consider the song "East to West" by Wolves at the Gate. I recognise that this kind of music is far from calming to most people. It is most definitely within the realms of that which would be considered "demonic" by hardcore Pharisees. Yet, on more than one occasion, God has used it to edify me. In the midst of temptation, I have been able to use this song to instead turn to God in praise. This song talks about the sacrifice Christ made for us. How He removed our sins from us as far as the East is from the West (Psalm 103:12). It emphasises the value of simply knowing God, and being known by Him, and the beauty of the forgiveness we receive by faith. Most prominently, it talks about the future we have because of this. The specific phrase "and I will join the quire to sing of His mercies and grace" in this song makes it very difficult to see sin as attractive. I often tell a story about how God used it to "one shot" Satan for me. I was very strongly tempted, and moved to sin, yet that part of the song came to my mind, and in one fluid movement, I instead turned to pray.
My guess is the same song would not have the same effect on you. That's just the way we are as a species. Our interests and emotions vary like the pebbles on a beach. Therefore, what worked for Saul might not work for me, and let's be honest, might not even work for you. Thus, the argument from David's harp doesn't work. Music still remains a doubtful disputation. Therefore, if you prefer the sound of a harp over the sound of a guitar, then you be what God made you to be. No one may judge you for it. In the same way, those of us who respond better to drums ought not be judged by those who prefer pianos. Let each man be convinced in his own mind, and no sin will be committed in our differences.