It's not the style of music that is sinful, but it what kind of response the music evokes when you listen to it.
First of all, we have to separate music from lyrics. When people say music, they often mean lyrics as well. And in genre's like "Rap", where there is virtually no instrumentation, and it's all lyrics set to rhythm, then we have to consider the lyrical content. In most commercial contemporary Rap, the lyrics are:
- pornographic, and
- not edifying.
Therefore, the act of purposely listening to this trash, or any music that has lyrics like this, be it Rap, Metal, or what not is, in fact, sinful; and the lyrics don't need to be pornographic. They can be:
- about suicide, or
- about committing murder.
If you listen to this stuff for enjoyment, then you are committing a sin.
Now, there is the subject of the music, itself, without the lyrics. In this case, you have you look at how does the body and emotions respond to the music.
- Does it make the hair on the back of your neck stand on end?
- Does it make you angry when you listen to it?
- Does it evoke rebellion?
If so, then you shouldn't be listening to it. It's not edifying. It's not sinful, in and of itself, but it could lead to sin. Nevertheless, it can serve a purpose. For instance, film scores often require music that evokes certain emotions in order to intensify the story line. In this controlled context, it's fine, but constant repeated exposure to music that makes you feel angry and rebellious, is not of God.
Having said all that; there is no such thing as a holy or unholy:
- key, or
We can get a bit a carried away. There is nothing wrong with well written Rock'n Roll that doesn't have offensive lyrics and that doesn't glorify sin. Classic heavy metal would fall into that same safe category, but when we start getting into rage, anger and rebellion, then the song not only has evil lyrics, but on a musical level, just stinks. I've been playing music for 35 years and can tell you it takes exactly zero talent and technical ability to play loud trash that has no melodic or harmonic value.
But again, I'll reiterate, we can't get carried away. There were certain "pinhead clerics" in the in the middle ages that actually tried to ban the playing of certain intervals and certain scales, calling them demonic. That didn't work out to well. There is nothing that makes one note holier than another or one instrument holier than another.