"Birds were singing in the morning."
smell・hear・taste・feel (non-visual sensory perception)
この はな は いい かおり が します ね
This flower smells good, doesn’t it.
くるま の おと が する。
I hear a car.
(In the) morning, birds’ voice was "heard" (by me).
I heard the birds singing in the morning.
Birds were singing in the morning.
「In the morning, the voice that belongs to birds was used.」
In most cases you can consider sentences with a ⒶのⒷ format to mark A's ownership of B. In this case, the bird's voice. There can of course be more chains of words belonging to one another, e.g., 私の彼氏の車（わたしのかれしのくるま・watashi no kareshi no kuruma）the car that belongs to the boyfriend that belongs to me; my boyfriend's car.
The correct english translation would be : "In the morning, birdvoice/birdsong was heard"
Because it is the way to say: (oto ga/koe ga) suru (おとが/こえが）する (a sound/voice) is heard
May be we coul also translate : "in the morning, we could hear the birds singing"
If you are using the word bank of Japanese and the beginning of the session starts with identifyng "naki" it comes as a surprise that DL wants you to use "koe" in this sentence and not to use the verb in the present continuous, which has been something of a feature for many of the previous sentences in this topic. Oh well, that's DL for you.
It incorrectly rejects virtually every attempt to provide an accurate Japanese translation of "Birds were singing in the morning". The only thing it seems to accept is the given "translation" meaning "The bird's voice was heard in the morning" (or more literally 'in the morning, the bird's voice acted'). Any attempt to indicate plurality of birds, the past progressive tense or other verbs meaning 'sing' ( 鳴く or さえずる) are roundly rejected.