Well for one, rock music they say is worldly isn't it? Yes, I know I sound hypocritical for saying I listen to rock music, but don't listen to Christian rock, yet here it is. I just don't like to mix Christianity with the world if I can, even though our best deeds are just filthy rags before the Lord. I hope I didn't offend anyone, and if I did, apologies.
Well, ultimately we all come to our own conclusions. But I see music itself as neutral and look to the lyrics to decide whether or not it's something I should or should not listen to. The Bible really makes little to no comment on music itself. Instruments are mentioned in the Psalms in the context of praising God (which, we use modern equivalents of those instruments today), but we are given no instructions on what music to use or what to avoid--which, to me, says it's not important.
Probably the most common argument against modern music is that it's worldly. So, basically, they are interpreting the Bible to say that anything that's popular must be avoided. That would mean movies, and many other things, are to be avoided to, too. (And there are some who do take this view, so I'll keep that in mind.) The Amish take this so far that they don't even have electricity or modern technology.
But, that doesn't make much logical sense to me. I think the verse is more about not assuming the attitudes of the world, not loving sin, not living in sin, don't do things that are sinful, etc. Be different from the world in our attitude so that they know there's something different about us.
But if being different from the world is to be defined as avoiding something just because it's popular--that really doesn't say much, does it?
Some might say that because rock music is often used to present wrong messages, that we need to avoid it--I guess in context of avoiding the appearance of evil? But no one's going to think you're a druggie or anything like that just because you listen to rock music, so personally it doesn't appear that there's any danger of appearing to be "evil".
And then there are those who claim that studies say rock music is unhealthy--mentioning a study where it said that rock music kills plants and stuff. I don't remember what I read because it's been a while, but when I looked a little further into this I saw they had skewed the truth to some degree. It seems that any source with a clear agenda will do this. I recently had someone tell me that some studies say that the beat in rock encourages violence because it brings out the baser instincts--I haven't looked into this, but it honestly doesn't make much sense to me. I have seen no evidence of this, and if it is true, it doesn't appear to be so to the extent that it's actually dangerous.
If anyone wants to look more into this for themselves, look at lots of sources. Take the ones who appear to have an agenda with a grain of salt. You rarely get the truth just by looking at one source, or just ones you agree with. You may not get the complete truth anyway, but you'll at least be more informed.
Since the Bible really doesn't say much about music, all we can really do is speculate about this issue. Both sides have good points to consider, and maybe it's because I come from a family who raised me to not listen to rock music that I try to be careful to examine the types of messages that the songs are giving.
Anyways, that's just my opinion. You may disagree, and that's cool. I'd never learn anything if everyone agreed with me all the time.