俱舍論記 13.159

13.159

「有外大種為因發聲」者，答。有外大種為因發聲，故有聲也。雖有內大為因發聲，外顯偏說，或可影顯。

Regarding "Having external great elements as causes, sound is produced": this is the answer. Because external great elements serve as the cause to produce sound, sound exists. Although internal great elements can also serve as the cause to produce sound, the text speaks exclusively of the external to be conspicuous, or perhaps it implies [both].