Camy &c. Your first choice is: Fatout, Paul, Mark Twain Speaking. Iowa City: University of Iowa Press, 1976. This will be reprinted at year end; this is the single most comprehensive (but not completely so) compilation of MT's lectures. Fatuout's Introduction is not to be missed! I would think tracing those actors & performers who played MT in the first decades after his death might uncover some audio files of persons who actually heard MT speaking. Delivery is only partially a function of accent; for SLC timing was everything! Gordon Snedecor in Portland, Oregon