Ok, I am still trying to sort this one out myself: Aufhören (to stop an action) is a different verb than Hören (to hear/listen). It's a answer, but not a good explanation.
Thanks for posting Emanuel's blog on aufhören. After reading it i wonder if DL's use is correct. Emmanuel says that the verb is used with regards to an action which you have control; not someone else. Si to command someone else to stop would aufhören be the correct verb to use.
I am extremely frustrated with Duo Lingo. The unit is on prepositions not separable verbs. I understand that prepositions are a part of separable verbs but it reslly is a different concept. Out of nowhere comes aufhören. No context no explanation. This is really bad teaching practice.