I seem to remember a recent thread about using Arduino to drive a graphic equaliser display, and that would give you the logic you need to detect the incoming sound level. I don't know whether you want to detect any particular type of sound or just trigger on a general 'loudness' - I suppose it would be possible to add an external band filter if you want to only detect sound in a particular frequency range. There is a shield which can be used to play pre-recorded samples, forget the supplier but it's been mentioned here in the past and I'm sure a Google search for Arduino sound shield would locate it for you.