as far as work songs go , I believe the earliest song that is known is the shadoof song of the Nile, Shadoof being the basket mechanism used for irrigation. its maybe 3000 bc.
there is an oddball theory, but one that appeals to me, that there are ancient recordings of in existence if only we could play them back, namely in pottery that was thrown on a wheel which in a similar fashion to the gramophone recorded some sounds as it was being made. It has been proposed that the recordings" if any may be read back with a laser device. cheers petr.