Looks like they used the plural version of hat. I don't know the singular version of hat, but it might be Hut? It might be Hut is to Hüte as Buch is to Bücher.
I'm not sure what the plural of hat is but from this context I believe it's Hüte without a indefinite and definite article.
I didn't hear any sound!!!! I think it is not my problem, because I heard all except this! =_=
Tapping didn't work and when I used the keyboard it didn't allow me to choose the Umlaut...Duolingo...what have you done? I have not used it in a while and the changes don't seem to work too well. :(