Final answer:
The figurative language used in the given sentence is a simile, which compares the difficulty of speaking words to speaking with a mouth full of chewed biscuits.
Step-by-step explanation:
The sentence 'Say goodbye to them?' he repeated, sputtering out the words as if his mouth was full of biscuits that he'd munched into tiny pieces but had not actually swallowed yet, employs figurative language, specifically a simile. A simile is a figure of speech that compares two different things in an interesting way, using the words "as if" or "like". In this case, the comparison is made between the way words are being sputtered out and the difficulty one might have speaking with a mouth full of chewed biscuits.