They could be testing out a weapon, perhaps not for military applications per se, but for spycraft and special operations.

Sure would be convenient to storm a guarded compound if all the guards were suddenly deaf. Similarly, it would be nice to use something like this as a perimeter fence of sorts. Slow exposure over time would eventually deafen the spies you suspect are surveilling your place.

The theory that the Russians were behind this, and not necessarily the Cubans, seems plausible if indeed this was a weapons test. Russia conducts a lot more SpecOps missions around the globe than Cuba does.