As for your first question, the Gemara says that if you had another way to go and you chose to travel that way, you are a Rasha. Obviously, we are talking about what is Assur to see.
And, to your second point, if you have to go there, then it would apply there, too. If, however, you had other places or options and you chose this one, that would be the Issur that the Gemara was talking about.
You differentiate between a view on the side and an inevitable sound. The distinction is true, but we don’t find such an Issur. Keep in mind that the original Din of Kol Isha is found by Brachos and Krias Shma. The point of having another way is that you specificaly chose this way, and you are conscious about the fact that you went down such a road. If you had no other way, than the problem doesn’t exist, it is not an Oness. The Gemara does not say that you have to be going there for a very important reason. The main issue is the fact that you know that you chose to go purposely that way.
I was only trying to differentiate beween your magnanimous example of a terribly inappropriately dressed someone, there to catch your attention. To that I responded that it doesn’t compare to the classic Hetter of Leka Darka Achrina, being that it is an aggressive, rather than passive, Erva.
This is all in pure Svara. In actual Psak, a Rov would most likely be Metzaref the fact that it is only a recording and you don’t know the person and it is usually low enough that if you’re not paying attention to it you won’t even notice it.