I have been in the barn in a heavy rain storm and have not found it to be loud. I've also stood in there during a major hail storm and had a conversation with my daughter. Harley slept right through it. At no point did the noise bother them in the least. Maybe it has something to do with the angle of the roof, the height of the building, or even the fact that I have a hayloft that may absorb some of the sound. As I said, the stalls are open all the way up to the roof, but running over my center aisle is a 24 x 20 hay loft (barn is 32 wide x 48 long). It is open on the sides as well, so my hay gets lots of ventilation. But even though it's only a partial loft, maybe it helps absorb some of the noise? In any case, I've been in much louder riding arenas, so I figure it's good for the horses to get used to the noise. They happily run in when it rains hard (Harley's a wimp about getting wet) and are happy to listen to the rain on the roof.
But again, that's why I specified in my original post that I am expressing what worked for ME in THIS climate. To those building barns, I suggest you visit a lot of barns in your area and talk to the owners about what they like, and what they would change. I learned a tremendous amount by doing this.
Another example, sliding doors for stalls. I was going to put some in, but they're expensive to get shipped here, and are not available locally. After talking to some very experienced horse people in the area, I decided against it. Many told me the doors would jam, get bent by a horse pushing against them, or just break down, and it was hard to find someone who could fix them. I went with regular swinging doors on the advice of these people. Easy to build (my contractor put them together easily), simple design means very little can go wrong, and if it does, it's simple enough that I can fix them myself. Out here, being able to do that means a lot. I also left the top part of my stalls open. There is a separating wall between the two stalls of course, and there is a partial wall on the front that prevents them from getting too close to each other. But they can happily stick their heads over the stall walls. One of my horsey neighbors saw the stalls and said that all he could think of was all the wood the horses were going to chew on. However, I know my horses. They are not chewers. They are stalled minimally. There isn't single tooth mark on a single board anywhere. Of course if someone had a cribber, or a horse that likes to chew wood, you'd have to come up with a different plan.