Final answer:
The literary device used in the sentence is a simile, which compares two different things using words like 'as' or 'like'.
Step-by-step explanation:
The sentence describes a voice being muffled as if there were biscuits in the mouth, which is a comparison using 'as if' to create a vivid image in the reader's imagination. This literary device is a simile, as it compares two different things by using words like 'as' or 'like'. A simile aims to offer a new way of looking at one thing by comparing it to something else that is typically unrelated.