We generally speak so fast that we can end up "eating" diphtongs between words. The "o" is there, but it's so short and weakened that it's barely audible. There is no rule for this, as it depends on your talking speed. If you try to say "siamo arrivati" close to the speed of the registration (note: normal conversation goes even faster!), you'll see that you'll automatically weaken the "o" sound.
Keep up the listening practice!