Spoken word is indeed a medium in the strict sense.

The Celt tradition being a prime candidate for that sort of thing (if you are interested in an attempt to consolidate the Irish oral tradition I highly recommend George Green’s “Hound”).

Gilgamesh might be a good candidate for the first probable one though, as the oral tradition is almost impossible to prove.

Alongside that I’d argue certain bits of the old testament feel like villainous monologues.