I'm confused by this sentance.
I had a friend write the sentence “미국이랑 한국애 들려요!” which I understood as “I’m stopping by the US and Korea!” However, once I looked up the base verb for 들려요, I discovered it is 들리다 which means to hear. So now I am very confused about this sentence. Does 들리다 mean something else as well or was the sentence misspelled and supposed to be 들러요 (conjugation for 들르다 to stop by)?

Question

Answer

That is not your fault but your friend's.
Many Koreans misuse 들리다 in the place where 들르다(stop by) should be used.

미국이랑 한국애 들러요 is correct.

Answer

Im korean so im not good at english nut i want to explain to you.
'미국이랑 한국에 들러요' 
This is correct sentence.
'들려요' is '들르+어+요'  
Base verb is 들르다
들르다 means go somewere.
들리다 means something is heard(this is the passive word), holding by somthing, rasing by something, cutted because its rased, etc.
Acconting of 어 is very hard for begginer so you can study about that.