Albert Barnes on Revelation 16:1

And I heard a great voice out of the temple. A loud voice out of the temple as seen in heaven, (Barnes on "Re 11:19",) and that came, therefore, from the very presence of God.

Saying to the seven angels. That had the seven vials of wrath. Barnes on "Re 15:1,7".

Go your ways. Your respective ways, to the fulfilment of the task assigned to each.

And pour out the vials of the wrath of God. Empty those vials; cause to come upon the earth the plagues indicated by their contents. The order in which this was to be done is not intimated. It seems to be supposed that that would be understood by each.

Upon the earth. The particular part of the earth is not here specified, but it should not be inferred that it was to be upon the earth in general, or that there were any calamities in consequence of this pouring out of the vials of wrath, to spread over the whole world. The subsequent statements show what parts of the earth were particularly to be affected.

(a) "angels" Rev 15:1,7