utterance, speak 分別?

Someone's "utterances" are the things that they say.
"Utterance" is the expression in words of ideas, thoughts, and feelings.

When you "speak," you use your voice in order to say something.

see p.1730 and p.1494 "Collins Cobuild English Dictionary for Advanced Learners" (third edition)

The word "utterance" is a noun, and "speak" a verb.