Unlike video, the only thing you really need to watch out for is anybody mentioning anybody's name. Anybody's pertinent personal information. Otherwise they were probably just wondering what it was all about what you were doing? They didn't see any camera guy. And sound in free air is free sound. It is really don't want to pick up personal conversations as you are correct, it's not nice to eavesdrop. Of course in doing so, you might be preventing a crime from happening? How about telling your football team to clobber the other football team's quarterback? LMAO and that came from a documentary crew that they knew was there. So, you can't fix stupid and they deserved what they got. Dumb floppers.You got to love that kind of crap. So, yeah, go eavesdrop in the football locker rooms with your recorder running.
The only time you actually need to have them sign a release for what you are doing is if you involve them, personally, in what you're doing. Of course I'm not a lawyer and for better peace of mind you might want to consult with an attorney? I think you'll find there should be no problems in what you're doing. Especially, if you're not doing it for any money but just personal use and enjoyment.
Did you know so-and-so was cheating on his wife?
Mx. Remy Ann David