In حُكِيَ بِهِ صَوْتٌ, حُكِيَ is a passive past verb.
صَوْتٌ: passive subject (nāʾib fāʿil), nominative with ḍammah, because a sound is what was imitated.
صَوْتٌ is an ordinary declinable noun here. Its nominative case does not conflict with the indeclinable status of the cited sound-word «غاق».
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Al-Aṣwāt — Sound-words, Their Two Uses and Grammatical Analysis
In حُكِيَ بِهِ صَوْتٌ, حُكِيَ is a passive past verb.
صَوْتٌ: passive subject (nāʾib fāʿil), nominative with ḍammah, because a sound is what was imitated.
صَوْتٌ is an ordinary declinable noun here. Its nominative case does not conflict with the indeclinable status of the cited sound-word «غاق».