金剛經纂要刊定記 6.97

6.97

云何不，結成疑也。意云：聲不自聲依色而發，既無所依之色，何有能依之聲？故成疑也。

"How could he not?" crystallizes the doubt. The meaning is: sound is not self-sounding but is emitted in reliance on form; since there is no form relied upon, how can there be sound that relies on it? Hence it forms a doubt.