In your experience would it be possible to generate such a beam of sound powerful enough to cause brain trauma, but keep it focused enough so that it is hardly audible outside the beam?
The human brain sits in a cavity of fairly specific volume, rigidity, shape and contents (mostly salt water). One could estimate (or better and easier, test for) the resonant frequency of such a cavity (fasten a sound-wave generator to a similar cavity and test its frequency response)[anyone want to volunteer their brain for science?]. But the human head consists of of a number of cavities (cranial cavity, the sinuses, the mouth, nasal cavities, ear passageways, eyeball, etc.) various bones (jawbone, tympanum, etc.) and collections of liquid (connective tissue, tendon, cerebrospinal fluid, etc.). Each part has its own resonant frequency and sensitivity to damage.There will be at least one resonant frequency for each cavity/object.
One could use a loudspeaker or sonic device at those frequencies (and multiples thereof) to remotely create resonant oscillations inside someone's head. Since you'd be using air transmission, the source would probably have to be powerful, but if the frequencies are out of the range of human hearing then they might not be noticed by someone other than the target.
If this is a weapon, likely lots of effort has gone into finding those parts of the human body most subject to failure.
Best bet for Cuban spies: get a dog. The dog will let you know when he's affected.
