He did not speak clearly and correctly.
The film is not interesting and instructive.
Do we parse it like this: He did not speak [clearly and correctly]?
Your two examples are slightly different in structure. Your first sentence could be rewritten as “He spoke not clearly and correctly” to match your second sentence. In that case, it could be parsed either as “…not [clearly and correctly]” or “…[not clearly] and correctly”, the same way your second sentence could be parsed either as “The film is [not interesting] and instructive.” or “The film is not [interesting and instructive].”
The context makes the meaning more clear, however, you might consider rewriting the sentence to be more obvious: “The film is neither interesting nor instructive.” or “The film is uninteresting but instructive.”