If you're in a situation where you can clearly hear other people's conversations, will it turn out to be an unethical act? This is something I'm currently faced with. Let's find out as we move along.
First point: Hearing other people's conversations can be a not-so-good thing to do but when you're confined in a small place with them and you couldn't help but hear what they say, that cannot be taken against you.
Second point: It is not your fault to hear people talking about confidential or non-confidential topics. More so, it is not your fault that you're situated near them and that you can hear them loud and clear. Unless, of course, if there's a defect with your ears then I'd say you're an exception.
Third point: If people talk about sensitive matters, the more appropriate place to do so is somewhere really private. That's if they find the topic too sensitive for other people to hear. Logically, if they're talking about it in public then they should have known beforehand the risks of getting heard by others. While there may be a law protecting communication per se, there is no such thing as a law against accidentally hearing other people's talks, is there?
Fourth point: For as long as you just keep what you've heard to yourself, then that shouldn't be a problem at all. But I understand that there are just instances when you feel the urge to spread the word to others. This calls for a debate within you. You should be mature enough to find out whether there's a need to tell it to others or not. Technically, I'd say you should keep it to yourself so as not to interfere with the communication that you're not even involved with but if it's something that needs to be exposed to the public then so be it.
Also, If you're wondering whether it is right to react within yourself about what you've heard or not then wonder no more. It's completely normal. Since you're given your own intellect, you would naturally have a say on what has been talked about even if you're not part of it.
I'd be very glad to hear some more tips regarding this issue if you have insights. Feel free to comment.