"Sammy's long speech," as his mother Jane Lampton  Clemens called it, was
acquired from her. Ron Powers in Dangerous Waters: "Jane  Lampton was in
many
ways the feminine version of the son who would in turn  render her immortal
as
Aunt Polly...She was small and red-haired, as Sam would  be, with small feet
and
hands; yet she was a passionate dancer, and her son  would be a dancer too.
She spoke in the soft, almost mannered drawl that Sam  would inherit and use
to
mesmerize his close-up listeners and his lecture-hall  audiences--the drawl
that could be mistaken for a drunken slur, and which he  once lampooned as
'my
drawling infirmity of speech.'" p 32

It may have been Sam's slow, measured speech that  contributed to Horace
Bixby's agreement to train him as a cub Pilot in April of  1857. I've read
somewhere Bixby's reaction to meeting Sam and his drawl was  distinctive.

From my readings, I would not call Sam's speech a  "Southern accent," as
much
as an idiosyncrasy he aped from his mother.  It  was simply one of the
things
he learned at his mother's knee.

David H  Fears