A world, I am building, had been flooded, causing a species of fire ant that makes ant rafts to evolve a collective behavior, ditching anthills and becoming an ant-hill (I'm not sorry). Over thousands of years, the flood waters receded and the clustering behavior of these ant-hills remained, creating a powerful, natural, biological army.
Over many millions of years and some complicated evolutionary history, I don't have room to explain, these ant-hills evolved into a collective consciousness that have shown the true limits of swarm intelligence. While each ant is just an ant, together as a whole, the collective is sapient.
As they do not breed and lack 'organs', so to speak, their species (Diluviumformica sapiens) consists of many smaller specimens, all working as a whole, communicating to each other using touch and pheromones, the larger whole communicates with each other through much more powerful pheromones.
They are virtually immortal and have met with humanity. To avoid war, humanity is attempting to communicate with these aliens, but we are failing. Humans communicate via sound and the ant-hills communicate via smell, it is proving difficult.
How can an audio based communicator like a human communicate with a pheromone based communicator like the ant-hill?