Did upticks in the alleged "prairie madness" (I've not heard of it though I have lived in North Dakota since 1989) coincide with certain times of the year? It would be most silent here in spells during the winter, but during those times there could be hardship from other factors too.
Were the native people living here perceived as suffering from "prairie madness"? What about people of European ancestry who grew up on the Plains? Were there differences, as regards people newly come from Europe? I would guess that people from Norway -- like a great many of the people who settled in the region where I live -- would be accustomed to long, quiet winters. The scenery is different, of course, but a prairie farm and an isolated Norwegian saeter might not be so different sound-wise. In either case there'd be sounds from domestic animals, I suppose. A settler here from a German village might find the sound-adjustment to be harder.
In the warm months there would likely have been plenty of cries and songs from birds living in the grasses. I hear coyotes from time to time although I live in town.
I'm not saying the article is bad, but there are questions....