Melbro you should have told them to be quiet during the safety briefing and made an example of them in front of the other passengers by saying extremely loudly something like "excuse me but your behaviour is putting the safety of the other passengers at risk as they cannot hear the safety demonstration I am sure that they would like to know what to do in an emergency situation even if you dont" - if it happens again at least you will be better prepared next time - all of us at some stage have had something similiar happen - its not nice but believe me when it happens to you once you make sure your prepared if it happens the second time !