Who are you, O man, to talk back to God? (Watson)

"I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion." So then it depends not on human will or exertion, but on God, who has mercy." Romans 9:15-16

God has a sovereign right and authority over man. He can do with His creatures as He pleases. Who shall dispute with God? Who shall ask Him a reason of His doings? "Who are you, O man, to talk back to God? Shall what is formed say to him who formed it—Why did you make me like this?" Romans 9:20

"Our God is in heaven and does whatever He pleases."
Psalm 115:3

"The Lord does whatever He pleases in heaven and on earth, in the seas and all the depths." Psalm 135:6

God sits as judge in the highest court, and is not bound to give a reason for His proceedings. "He puts down one, and raises up another." He has salvation and damnation in His power. He has the key of justice in His hand, to lock up whomever he will, in the fiery prison of hell! And He has the key of mercy in His hand, to open heaven's gate to whomever He pleases! The name engraved upon His vesture is, "King of kings, and Lord of lords!" He sits Lord paramount, and who can call Him to account? The world is God's house, and shall not He do what He desires in His own house?

"My counsel shall stand, and I will accomplish all my purpose"
Isaiah 46:10

"Allelujah! For the Lord God omnipotent, reigneth!"
Revelation 19:6

(Thomas Watson, "Body of Divinity")

A relationship involves talking back. Sometimes we speak to our LORD in love, concern and pure frustration. I have talked back to the LORD many a times when I am going thru something that I can't understand. As long as we keep our complaint to Him and not to someone else, I believe He understands and listens to our complaints. Always without fail after I make my complaints to him my words changes to "But you know all things, you are a holy God and whatever your doing with me keep doing it, even when I don't understand it. And then it goes into worship.

Psalms 142:2 I poured out my complaint before him; I shewed before him my trouble.

I think it all boils down to motives and intents. It depends on who is doing the talking back and why. If your murmuring is with others and not directly to the LORD, I think those words would ring loud and clear, "Who are you to talk back to God".