.... ga kikoeta. .... ga mai-asa kikoeru. "them" might be " ...の声 ( .... no koe )” ; 学生たちの声 , 鳥のさえずり ( chatting of them, chirp of birds)

You can use 聞く in those case. 聞く can mean to ask, though. So you have to read it from context. 聞こえる means "to be heard" or "to be audible". It's an intransitive verb, meaning that it doesn't take an object. "I heard them yesterday." can be 昨日彼らを聞いた。or 昨日彼らが聞こえた。 "I hear them every morning." can be 毎朝彼らを聞く。or 毎朝彼らが聞こえる。 I'm not a native speaker so correct me if I'm wrong :)