"그 소녀는 새 소리를 들었습니다."
번역:The girl heard a bird.
The girl heard bird. 를 답으로 제시하였습니다. 관사 a 가 꼭 필요한가요??
그리고 또, The girl heard bird sound, bird singing 이 더 자연스러운거 같은데 어떻게 생각하시는지요??
Both of your sentences don't make sense, number one. Two, yes. You have to add the word "a" to the sentence. And your second sentence "The girl heard bird sound, bird singing" is just... illogical. It's definitely not fluid. If you wanted to say that sentence though, you would say along the lines of "The girl heard a bird singing", etc.