Again the LORD spoke to Ahaz: As mentioned in the introductory comments on this section [7.10-25], the LORD is the speaker here, but he probably continues to speak through Isaiah (see verse 3). The LORD is mentioned explicitly, and there is no valid reason to change the text to read “Isaiah spoke….” Good News Translation handles this verse well by saying “The LORD sent another message to Ahaz.” This rendering can be justified on the grounds that the Hebrew word translated Again (literally “And he added”) indicates that the dialogue begun in verse 3 continues. That setting involved three participants (the LORD, Isaiah, and Ahaz). Bible en français courant has “The Lord added this other message for Ahaz.”

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .