Parsing a sentence about hearing a sound

Al-Aṣwāt — Sound-words, Their Two Uses and Grammatical Analysis

سَمِعْتُ صَوْتَ الْغُرَابِ — I heard the crow’s sound.
سَمِعْتُ: past verb plus the attached first-person subject pronoun in the nominative position.
صَوْتَ: accusative object with fatḥah; first term of an iḍāfah.
الْغُرَابِ: genitive second term with kasrah.

صَوْتَ bears accusative case because سَمِعَ governs it. This is not how the imitation «غاق» itself functions.