we are building a house with a large gabled room above the garage to be our music room. The total dimension is 39' long 24' wide and 15' high at the ridge line of the gable.
This will leave a listening/music room area of 29'x24'
I am a retired carpenter builder 45 yrs exp. So let me try to understand the situation better
I believe what you have is a bonus room above the 24 x 39 garage. Is this correct? Then the 15 ft high ridge is measured from the floor of the room and NOT the garage floor. is that correct? Apparently you have a very steep roof. What is the roof pitch? I could do better IF I could see the print. But at this point, I'm questioning IF your room is really going to be 24 x 29. The 29' will stay the same . But because of the slope of the rafters and knee wall, the 24' is going to change and be smaller IMO. This is based on what little info I have and my experience
Whatever the case, I would still run the speakers firing down the 29' way