It amazes me how people talk on an airplane. If you are having a discussion with your seat mate, there is no reason to make the conversation audible to everyone in the cabin with you. I think you have two reasonable options in this kind of situation:
1. Approach the people politely and explain that they can be heard by more people than they may realize.
2. Approach the FA and ask her to do the same.

My opinion, FWIW.