Project Mogul: Does upper atmosphere indeed have a sound waveguide channel?
More than a year later, I'm doing a preliminary literature survey (noting that infrasound propagation is used in nuke test ban treaty verification):


https://dx.doi.org/10.1016/0012-8252(77)90063-0 W.L. Jones. Waves in the atmosphere; Atmospheric infrasound and gravity waves, their generation and propagation. (1977)
https://dx.doi.org/10.1111/j.1365-246x.2011.04975.x Green et al. Infrasound radiated by the Gerdec and Chelopechene explosions: propagation along unexpected paths. (2011)
https://dx.doi.org/10.1111/j.1365-246x.2010.04925.x Groot-Hedlin, Hedlin, Walker. Finite difference synthesis of infrasound propagation through a windy, viscous atmosphere: application to a bolide explosion detected by seismic networks. (2011)
https://dx.doi.org/10.1029/2011jd017014 Evers et al. Anomalous infrasound propagation in a hot stratosphere and the existence of extremely small shadow zones. (2012)
https://dx.doi.org/10.1121/1.1903454 Liszka. Long-distance propagation of infrasound from artificial sources. (1974)
https://dx.doi.org/10.3390/ani3020349 Kelley, Garstang. On the possible detection of lightning storms by elephants. (2013) (courtesy of casey)


... and a ton of others. When I have enough time, will study. Others may surely use the leads I collected to post an answer before that.

Things to keep in mind: anomalous propagation, whether acoustic or electromagnetic, below the surface or above it, is based on speed inversions. Whenever there are countervailing trends in salinity (for water)/water content (for air), density, and temperature, natural waveguides may exist.