Cornelsen has a fantastic free app called PagePlayer . You can download it on iOS or Android.
Just three months ago, the German language had felt like an unclimbable wall of compound nouns and confusing cases. But today, Elias had his "secret weapon" downloaded and ready. As the suburbs of Brandenburg blurred past the window, he played track 42—the one about two-way prepositions. "An, auf, hinter, in, neben..." the narrator recited. Cornelsen Grammatik Aktiv A1-b1 Audio Download