I've been reading "Newspaper Articles by Mark Twain" for Librivox.org  
and came across the fact that Samuel Clemens apparently first spoke  
before a large audience on January 27th, 1864.
On the following day, in the Territorial Enterprise, datelined Carson  
City, he wrote:

"I delivered that message last night, but I didn't talk loud enough— 
people in the far end of the hall could not hear me. They said "Louder— 
louder," occasionally, but I thought that was a way they had—a joke,  
as it were. I had never talked to a crowd before, and knew none of the  
tactics of the public speaker. I suppose I spoke loud enough for some  
houses, but not for that District Court room, which is about seventy- 
five feet from floor to roof, and has no ceiling. I hope the people  
will deal as mildly with me, however, as I did with the public  
officers in the annual message. Some folks heard the entire document,  
though—there is some comfort in that. ..."