When will Hashem speak with him?
Rashi: When I will want to speak with you [I will send you to them].
Radak: When I will finish speaking with you. This was when "va'Ya'al me'Alai ha'Mar'eh Asher Ra'isi" (11:24).
Malbim: This is after all the actions that He commanded him in Perakim 4 and 5.
How will He open his mouth?
Radak: He will permit him to speak to them. Chazal said 1 , this is when "v'Yad Hashem Haysah Alai ba'Erev Lifnei Bo ha'Palit? va'Yipasach Pi" (33:22).
Malbim: He will not rebuke them like one who rebukes Musar from his Sechel. Rather, He will tell them a brief saying (refers to 3:27:4:2).
The only earlier source I found for this is Rashi there. (PF)
What will he say in the name of Hashem?
Rashi: All the words of the Shelichus.
Why does it say "ha'Shome'a Yishma veha'Chadel Yechdal"?
Rashi: This is not part of the Shelichus. Rather, Ru'ach ha'Kodesh tells the Navi, tell them My Shelichus. One who listens will listen, and one who refrains will refrain. I know that not all will listen, "Ki Beis Meri Hemah."
Malbim: Hashem told him to tell them that one who hears rebuke of Musar, he will merit to hear; he will live. One who refrains from hearing Musar, he will cease from the world.
