That's interesting, I remember back in high school band I also used to think it was the gym being so open that affected the sound the most. I imagine that is a common misconception. I remember my band teacher used to freak out about the sound when we would rehearse there before the concert. I wonder if she knew much about acoustics. Now that I've learned a little more, the fact that the sound absorption is the issue makes perfect sense. No wonder our actual concerts usually turned out fine.