How do you handle telling people to stfu *nicely*?

I have excellent hearing and I can get sensory overload easily. I need to find ways to ask people to keep it down without seeming like a total bitca (bonus points if you get that reference 😉).

I'm a Tour Guide, and yesterday, I was giving the tour introduction while a group of 3 people in the front row were talking at regular conversation level voices. It was sooooo distracting. I tried talking over them - I was even on a mic! But finally, it got to be too much, and I said (into the mic!), "You guys are being SO LOUD!" They shut up, but what I said kinda killed the vibe in the room, and I think other people were even afraid to laugh too much at my jokes, lest I scold them too. 😆

Those people were the rude, inappropriate ones, but I still wish I could have handled it differently. Any suggestions for what to say to people when I'd like them to be quieter or even silent??