"Sound is eternal, because it is caused" commits the fallacy (hetvābhāsa) of:

The reason 'being produced' actually proves non-eternality, the very opposite of the intended probandum of eternity.

Whatever is produced, like a pot, is invariably found to be non-eternal.

So this middle term establishes the contradictory of what the thesis claims.

A reason that proves the opposite of the sadhya is the contradictory middle.

This defect is viruddha hetu.