Ṣādiqatā is a feminine dual adjective, referring in context to the animal’s two ears even if the ears are understood from the surrounding description. Samʿ and al-tawajj us further specify the quality of attentive hearing.
In li-hajsin khafiyyin, li- is a preposition, hajsin is genitive, and khafiyyin is its adjective. Aw li-ṣawtin coordinates the alternative louder sound. The syntax distinguishes two auditory situations while keeping the same attentive listener.