This is a very difficult question for me to answer. While I think it is totally appropriate to ask, in essence, someone to keep it down, I struggle with this for a few reasons. One of them being that I have a very loud laugh. I try to stifle it at times, especially in public, but sometimes it overcomes me and I seriously cannot help it. Another issue is that my mother is unreasonably loud at times, and despite many family members trying to, in private, let her know she gets too loud, she argues that she is not being loud at all. She becomes very defensive if you even mention that she was a tad loud and might have been embarrassing, and then she does absolutely nothing to fix it in the future. I have even tried to explain that she makes everyone around her uncomfortable, but she insists that she has done nothing wrong.
I think, in some aspects, you risk either embarrassing someone (me) or putting them on the defensive (my mom) when you ask them to keep it down, especially because someone's loud may be another person's reasonable tone (and that isn't even considering either my mother or me). I think in public places that are bound to get a little loud, ie a train station, a cafe, or a restaurant, it may be inappropriate to directly ask someone to keep it down. In public places that are expected to be quiet, ie a church or library, I do not think it is inappropriate. In the loud places, I think the only thing you can do, really, is speak to the people who work there.
I, too, have a loud laugh that can come out explosively when I think something's funny (and I can see humor in just about everything). Through training and voice modulation I've rarely been told, as an adult, that I'm speaking too loud, but my laugh is spontaneous and I live in an area of quiet talkers and reserved laughers. So I've been told on more than one occasion 'indoor voice, bloo' when I'm laughing...even in my own home (by a guest). I've felt condescended to but it wasn't worth an argument and I'd prefer not to make already uncomfortable people even more uncomfortable (friends anyways).
I've been told by many a local, "I just love your laugh," which can mean anything from, 'I love your laugh,' to 'You laugh too much and too loud, do shut up please,' depending on tone. I asked DH awhile ago if I should try harder to stifle my laugh since even speaking with enthusiasm is rare in my parts, much less laughter and he said, 'no, they need to be shook up by different personalities,' which I really appreciated. Not to say he won't signal me when I'm too loud, just that he doesn't want me to stifle my personality.