I was actually an offender of this once, I'm ashamed to admit. I got upgraded next to another FlyerTalker I hadn't seen in a while, and we were having a grand time catching up. We weren't thinking much of our conversation until the gentleman seated behind us asked us very politely to keep our voices down, and we of course adjusted immediately.
I actually appreciated the comment - I don't want anyone to be inconvenienced by me on a flight, and was more than happy to adjust my volume (and then quickly wrap up my conversation entirely) to accomodate the request.
I say go for it and politely ask the offender to speak more softly - they may be rude about it, but odds are just as good that they'll be embarrassed and apologetic like I was in that situation.