Daewoo DVD + VCR DF4501PN



I have recently purchased this machine, which is fab, untill now. I am trying to copy some tapes onto DVD, I have done 3 so far with no problem, when I went to do the 4th, it had no sound. I have checked this video on other machines in the house and it is fine. I know I am missing a small detail but do not know what. Would anyone have any ideas please, as I cannot find anything in the manual.