Scootaloo panted hard, her hair was a mess. Her cheeks glowed red and sweat dripped off her face. Her lips mouthed the words ‘stop’, ‘please’, and ‘mercy’, but no sound came out.

Maybe, "but no words came out." would be more fitting? She would be really out of breath at that moment.

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I think either way work well enough, or maybe "but nothing came out"? All convey the general idea.
Thanks for reading too!