Rather, omit the supplied word “where.” The idea is that, despite the fact that the heavens have a language of their own (see vs. 1, 2), their voice is inaudible; it does not speak to the ear, but to the understanding heart. Addison sang:

“What though in solemn silence all

Move round this dark terrestrial ball? ...

In reason’s ear they all rejoice

And utter forth a glorious voice.”