Will make sense if you ever watch Animal Planet or The Dodo on Youtube. Plenty of singing dogs, dancing dogs, and in Korea a dog that helps a restaurant owner greet and handout menus.
를/을 is used to mark the object of a verb; if the dog were singing a song, song would be marked: 개가 노래를 노래합니다. In this case the dog isn't singing anything particular, just singing (i.e. the verb is intransitive and has no object), so there is no 를/을.