The short answer is: of is a very flexible word.
I will speak on love.
I will speak about love.
I will speak of love.
These three expressions mean essentially the same thing.
You already know about speak about, but:
- To speak on X can mean "speak on the topic of X" – although, when it comes to using on, it might be more common to hear, "I will give a talk on X."
- To speak of X can also mean "speak about X," although it has a slightly more smooth and literary feel to it.
If anything, speak of seems to be more prevalent than speak about, at least in written texts – if you want to believe the ngram: