What format are the videos / what editing package are you familiar with?
Most have the ability to edit the volume level of the sound track and to add extra sounds.
I haven't done it but thinking out loud you could drop the recorded sound volume at that exact point and add a beep at that point, then resume the volume of the recorded sound. In something like Pinnacle Studio I reckon it'd be fairly easy.
Is your video taken on a mobile or something by any chance? I'm more familiar with editing DV footage in say Pinnacle than editing a .MP4 file or whatever.