Henri,
Have you checked in Slide show - setup show if the show is browsed in a
kiosk, if so activate the radio button next to one speaker.

Also have you tried pressing F5 which should start the show from the
beginning.

Or maybe there is a transition time applied to slide one, look in slide
transition task pane and see if there is a timing set next to automatic.

I am guessing here, as you have given no clue as to what you have already
tried to play the show.