I'd like for this to be how a small species communicates with a giant species. I was assuming a series of tunnels would carry the sound like a pipe organ.
I'm not certain the effect is quite what you're describing, but this reminds me of the Hypogeum of Hal-Saflieni.
The structure is designed in a way that produces strange acoustic properties. If a person speaks while standing in the right spot in a certain room niches in the walls amplify the sound; they can be heard throughout the entire structure.
I'm thinking an alphorn or even a blowhorn design. The opening where they speak into is extremely small, just big enough for your creature's mouth, and then it's a smooth tunnel until the opening. I don't know enough to comment on whether the tunnel has to gradually enlarge or enlarge all at once like an alphorn, but I do know it has to be smooth all the way through, any imperfection is going to damage the initial message.