Don't mutter to yourself under your breath, but speak clearly and distinctly.

And Dave would mutter an "expletive deleted."

Don't mutter to yourself under your breath, but speak clearly and distinctly.

For he hath heard your murmuring against the Lord: but as for us, what are we, that you mutter against us?

She was often observed to raise her hands to her neck, as if in search of the ribbon that had been taken from it, and mutter, in surprise and discontent, when she could not find it, “It was the link that bound me to life.”