His mouth worked for a moment, but nothing came out.

His mouth worked as he thought through what I had said.

Her mouth worked as though trying to form a word.

The woman's mouth was working as she tried to find words.

His mouth worked several times before any sound came out.

His mouth worked a couple of times before words came out.

His mouth worked up and down, but he couldn't make a sound.

"Your hands and mouth can work at the same time."

His mouth worked a little and he looked ready to kill me.

Her mouth worked, but whatever she'd been meaning to say couldn't force its way out.