If it is the rocker cover gasket then it shouldnt cost more than £10 for a new one and its reletavely simple to fit although I have never done one on a Fiat before I have done one on my old Clio and it was a case of airfilter housing off, 8 bolts removed and a clean lint free cloth to clean up muck from around the seal, then place new gasket in place and reverse the process.
As for the rear speaker wiring, I dunno about the Punto but the Cinq didnt have any wiring there so we had to lay new wire from the ISO all the way down the trim and under the carpet, not a hard job just takes a bit of patience and small fingers at times