Ethically, if you are uncertain about attributing sources in a speech, your text suggests

when in doubt, document all of your sources in a speech.

Explanation:

It is the ethical responsibility of a speaker that he should provide honest and unbiased information to his audiences. And it is not a one-time event but a speaker should consider ethics while public speaking, every time. A public speaker who avoids plagiarism and mention the source of information increases his credibility. But in case of doubt, documentation of all the sources becomes necessary.