صَوْتُ الْغُرَابِ is an ordinary iḍāfah phrase meaning “the crow’s sound.”
غاق is the word used to mimic that sound.
The noun صَوْت can have normal syntactic case in a sentence; the quoted sound-word is a different grammatical item.
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Imitated sound versus ordinary noun
Al-Aṣwāt — Sound-words, Their Two Uses and Grammatical Analysis
صَوْتُ الْغُرَابِ is an ordinary iḍāfah phrase meaning “the crow’s sound.”
غاق is the word used to mimic that sound.
The noun صَوْت can have normal syntactic case in a sentence; the quoted sound-word is a different grammatical item.