Not a sound was to be heard in the concert hall.
I doubt that algún is correct after the noun. Wiktionary says: “The form algún is used only before and within the noun phrase of the modified singular masculine noun. In other positions, alguno is used instead.”
I agree, I’m just ignoring this sentence.
In fact the sentence has been fixed on tatoeba, but hasn’t been fixed on clozemaster yet. No se puede oír sonido alguno en la sala de conciertos. - Spanish example sentence - Tatoeba