Speak
Like "say," the word "speak" refers to thoughts and feelings moving from our more internal spiritual levels to our more external ones -- and ultimately from the Lord, who is in a sense the most internal spiritual level of all. This is generally called "influx" and "perception" in the Writings, meaning they are thoughts and feelings that flow in in a complete way from the Lord, rather than being things we have to think about and figure out. On a number of occasions "speak" and "say" are used together; in these cases "speak" refers more to intellectual instruction in matters of thought and "say" refers more to feelings and affections that flow in directly.
Jeremiah 22:5-6
5
But if you will not hear these words, I swear by myself, says Yahweh, that this house shall become a desolation.
6
For thus says Yahweh concerning the house of the king of Judah: You are Gilead to me, [and] the head of Lebanon; [yet] surely I will make you a wilderness, [and] cities which are not inhabited.