Utterance is the noun form of utter (verb), which means to speak or say. So an utterance can be any vocally produced sound -- it doesn't even have to be a word. Technically, a burp could be considered an utterance.
A sentence (in the context you mean) can be written or spoken. It's simply a group of words that expresses a complete thought or idea. Sentences typically contain a subject and a predicate containing a verb.
I'm not sure if that answers your question, but hopefully it helps!
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