Abstract: The Upano Valley of Amazonian Ecuador, recently revealed by LiDAR survey and published in Science (Rostain et al. 2024), comprises over 6,000 rectangular earthen platforms distributed across 300 km² connected by straight roads. Conventional interpretation attributes these structures to residential and ceremonial functions. This study applies archaeoacoustic analysis to the published platform dimensions, demonstrating that the standard platform (20m × 10m) encodes a perfect octave pair at 8.57 Hz and 17.15 Hz, the large ceremonial platform at Kilamope (140m × 40m) encodes frequencies of 1.23 Hz and 4.29 Hz, and plaza groupings of three and six platforms encode 2.86 Hz and 1.43 Hz respectively — all within the infrasound range of 0.5–20 Hz. Cross-civilizational harmonic verification confirms all six dimension-derived frequencies align with integer harmonics of the established F#6 carrier frequency (1479.97 Hz) at errors below 0.35%. Critically, published seismoacoustic monitoring data confirms that the adjacent Sangay volcano — one of the most continuously active volcanoes in the world — produces quasi-continuous harmonic tremor in the 0.5–4 Hz range, precisely matching the frequencies encoded in the Upano platforms. The volcanic andisol substrate and Upano River corridor provide natural acoustic transmission pathways. These findings suggest the Upano Valley represents the largest engineered acoustic instrument yet identified — 6,000 resonant platforms tuned to their local volcanic infrasound source, functioning as a distributed ground-coupling network for seismic monitoring and long-range signal transmission. The culture's disappearance around 300–600 CE, previously attributed to possible volcanic activity, is reinterpreted as network failure following disruption of the volcanic infrasound source.