(I'm the trailing spouse to a US diplomat and know some of the people in Cuba -- if anything, the media coverage is underplaying how weird this is.)
My understanding is that nobody is entirely clear whether this is permanent or not. Some people have recovered quickly and others are still having lingering effects. Several suffered a concussion, which just doesn't work -- I work for an audio simulation company (think jets and helicopters) and there's just no way audible sound or even infrasound can do this.
OTOH, there is a theory that guided infrasound can do serious tissue damage, which was possibly the cause of Dyatlov Pass[1].
There are numerous drugs that can cause repetitive auditory hallucinations, mild brain injury, and CNS damage. A sufficiently advanced actor could easily try to sophisticate a new substance that would enable this kind of attack without showing up on toxicology screens.