It means that the speech was made in a lively, interesting, engaging and merry manner.
Example: We had a light-hearted laugh when we saw our puppy chasing its own tail.
Just to complete the discussion, we do something with a "heavy heart" when we are feeling sad/melancholic.
Example: It is with a heavy heart that I announce the passing of our school Principal this morning. He was involved in a car crash and was killed immediately.
Hope this helps.