What did Frederick Douglass regularly speak to his audiences about?

Not surprisingly, the major topic on which Frederick
Douglass spoke to audiences was slavery.  In the time after he escaped from slavery,
Douglass's public life was mainly dedicated to the cause of
abolition.

In his speeches, Douglass talked about slavery
in a variety of contexts.  However, slavery was always at the heart of what he was
discussing.  So, when he talked about his attitudes towards the Constitution, he talked
about how he felt about the fact that it allowed slavery.  When he talked about the
Civil War, he did so because of the impact it would have on
slavery.

So the short answer here is that he was an
abolitionist and talked about slavery in most of his speeches.