I think it depends on what the speaker means. "Most of you would do the right think if you knew," indicates that the speaker is referring to the audience for the entire sentence; meaning that the audience would do 'something' if the audience knew 'something'.
To me, "Most of you would do the right thing if they knew," suggests a third party involved. For example, "Most of you(audience) would do the right thing if they(police/figure of authority/etc.) knew."
In my experiences, when addressing an audience, if the speaker uses a pronoun, (i.e. they, he, she, etc.), then it hasn't typically referred to anyone in the audience unless they say, "(Specific audience member(s)), they/he/she..." and so on. So, if the second "you/they" is referring to the audience specifically, and not a third party, then the correct sentence would be, "Most of you would do the right thing if you knew."