Why should the robot care about the honking? It should already have way more data via sensor input than the human doing the honking has long before the honk ever occurs.
Even ignoring that, I don't think I've ever seen a situation in which I think a horn actually helped in a near-accident situation. It tends to just be the thing someone honks in anger after the immediate danger is already avoided. IME (as someone who has never been in an accident but has seen quite a few near misses and accidents on SoCal freeways) usually once the horn comes into play it is either too late and the accident occurs anyway or the horn is honked after the fact as a "fuck you" to the other person.
Actually it should care about signals. Maybe not honking, but there are cases like emergency vehicles that you react to. I'm not sure what's the standard reaction in other parts, but in the UK when you hear / see an ambulance or a police car behind you, you stop on the side of the road. Sometimes partly on the pedestrian area. Would the self driving car do the same to let the emergency vehicle pass it? Unfortunately that's unlikely.
Even ignoring that, I don't think I've ever seen a situation in which I think a horn actually helped in a near-accident situation. It tends to just be the thing someone honks in anger after the immediate danger is already avoided. IME (as someone who has never been in an accident but has seen quite a few near misses and accidents on SoCal freeways) usually once the horn comes into play it is either too late and the accident occurs anyway or the horn is honked after the fact as a "fuck you" to the other person.