Is sound room on the attic bad???

6bq5 has the right idea. The none parallel ceiling is a good thing, not a bad thing. Any rectangular room will have three big standing waves associated with the three dimensions of the room and the wavelengths generated by the speakers. If you assume sound travels at an even 1000 feet per second, a room 20 feet long will have a 50 hz standing wave (also multiples ie 100, 150 hz etc). If the walls were not parallel then you would remove the standing waves and only have to worry about reflections. Many attics are not finished. If you are thinking about putting in wall board or sheet rock, don't. A room with bear studs and insolation would approach an anachoic chamber and minimize reflections.

The ideal sound room does not have parallel surfaces, and has different textures to cancel out standing waves, early reflections and echo points. With the ceiling being peaked, you are off to a good start, as the ceiling and the floor are probably the two largest planes of your room. I agree w/ Chris, set it up and listen.
Happy Listening.

What are the dimensions of your room? How high off the floor does the roof slope, and what is the peek height? It may be that this roof could work to your favor but I need more information.

If I understand corectly your attic has a concrete floor and the roof angles upward on both sides to a peak? Does that mean that it has no vertical sides or that the sides are short instead of the usual 8 feet?

If the side walls are short and the center peak of the roof is high (sort of like a "A" frame) you could have an excellent room without many room nodes.

Acoustic treatment to deal with early reflection or overly bright may be needed.

Good Luck!!!! Chris

Patrick, I think it also will depend on how you expect your music to sound. If you strive for the absolute sound ala TAS, i.e. as close as possible to actual recording event, you will probably be in big trouble and will have to think of putting in a ceiling and dampening it at the critical points. If I were in your shoes however, I'd just set up my system and sit down and listen, If I like what I hear, fine. If not, see the above..
Good luck and regards,