I have to chime in. I wouldn't try to change a thing in him. Compromise may work much, much better in this situation.
Some people have a strong creative spirit. Some never know about it until something captures their interest and things just sort of "click". For me, it was creative writing and photography. Photography is now my career, until God leads me to do something else. I'll tell you right up front that my wife really criticized me about my photography, and it was because she was angry about how much time and effort I poured into it when I was learning the craft. Looking back, YES, I absolutely did spend too much time with it at times, for sure. But when she criticized all of it, it really hurt me very deeply. When I gave up photography for a time to pour myself into my corporate career, I died inside because I had no creative outlet. And then problems were compounded.
I really identify with the the problem you are facing. If he's really into the music, please, please do NOT tell him you think he stinks at it. Try to compromise - get him to move the computer to another room, or if you can afford it, buy him his own computer. Get him to agree to limit his time and expenses with this hobby by giving him a "bank" of hours to use each week. If he gets an hour a day on weekays (5 hours a week) and he gets really creative one night and needs three hours, let him have it. Then he only has two hours left for the remainder of the week. 5-7 hours per week is a really good start.
If you love this guy, show it by supporting him. Perhaps buy him a gift certificate to attend a workshop, or a new software program. Believe me, if you do this and he knows you don't exactly like his style of music, he'll think the world of you - because he knows you did it out of love, even if you have to wear earplugs. Walking up behind him, putting your hands on his shoulders and whispering into his ear, "Now that sounds nice - good work,honey" would probably blow his mind.
My wife loves to make baby blankets and throws. I personally hated the idea. I thought it was the biggest waste of time. We still have bolts of cloth in our closets! But I thought back to how things went with my photography "addiction", and went out and bought her a really cool sewing machine and some books and a ton of supplies for Chistmas last year (had absolutely no idea what I was supposed to buy). I really see the joy she gets when she turns out a beautiful blanket. And they make nice gifts.
I will warn you that if he really likes this music thing and you make any attempt to kill it, he will either sink deeper into it as an escape or become very bitter.
Also, if he does have a creative edge, he may find that the music route is not satisfying him, and move onto something else. Just remember that pursuing creative interests in moderation (unless it's a career thing) is like feeding the soul for some people. Creative types like variety.
Finally, I hang out with creative professionals (photography, art) and hobbyists. And I find some of them use it as a temporary escape from their normal daily lives and responsibilities. If you establish boundaries in advance, and let him have free reign within those boundaries, tensions should be eased a good bit for both of you.
He enjoys composing music using a computer program. Enjoys is probably a understatement. Much of his free time is spent working on his music. I am not impressed with his music. I am slightly annoyed when he insists on working on it (in the family room) while I'm watching my favorite television show (once a week). I find it more than a little selfish that he wants the children to be silent while he's working on his music. I don't encourage him in this endeavor because I just don't see the point, neither do I discourage it because it seems to bring him joy. My point is that it is really causing division. There are no subtle ways to say, "Get a new hobby!" 
