The 2020 IOM report on "Havana syndrome" only considers RF (radiofrequency) as the kind of directional phenomena/weapon that could have caused the reported symptoms.
They've not mentioned anything sonic (i.e. transmitted as air pressure changes) instead of electromagnetic as being worth considering as alternative explanation, although they do go over a number of alternatives including chemical, infectious, and psychosocial.
There is no explicit reasoning given why they didn't bother considering sonic weapons at all.
On the other hand, one of the experts consulted, in an interview with WebMD, didn't totally exclude some kind of ultrasonic device:
James Giordano, PhD, a professor of neurology at Georgetown University and senior fellow in biosecurity, technology, and ethics at the U.S. Naval War College, has consulted for the government on the syndrome and has access to data and medical records from the attending doctors in Havana. He was asked to help explain the illness and what might have caused it. [...]
Giordano dismissed the possibility that the effects were due to either accidental or deliberate exposure to a toxic chemical, pesticide, or drug. No traces of any such agent were ever found in the homes or bodies of those affected. Instead, he thinks the most likely cause was some kind of mechanical device emitting ultrasonic or microwave energy. "This is the primary possibility with high probability," he says.
The known anti-personnel microwave weapons developed for the US military exhibit a burning sensation on the target/victim, so questions remain as to how the alleged RF attacks were carried out without this effect predominating. The main working theory for how RF caused the sound symptoms seems to be the Frey effect (it's mention a dozen times as least in the IOM report), but this has been studied less and hasn't been weaponized to date, at least according to what's publicly known. How the longer term symptoms were caused seems even less understood, e.g. Giordano said that
exposure could potentially create bubbling in the fluid inside a person's ear, and those tiny bubbles could cascade through the blood to the brain, causing damage similar to the decompression sickness some scuba divers experience.
I have no idea how plausible this particular mechanism is, in the opinion of other experts.
At least a couple of US microwave weapon experts have discounted that loud sound could be reasonably be experienced via RF without burning or even deadly injuries occurring first:
There is no way the ray gun could deliver sound loud enough to be annoying at nonfatal power levels, says Kenneth Foster, a bioengineering professor at the University of Pennsylvania who first published research on the microwave auditory effect in 1974.
”Any kind of exposure you could give to someone that wouldn’t burn them to a crisp would produce a sound too weak to have any effect,” Foster says.
Bill Guy, a former professor at the University of Washington who has also published on the microwave auditory effect, agrees. ”There couldn’t possibly be a hazard from the sound, because the heat would get you first,” Guy says.
Guy says that experiments have demonstrated that radiation at 40 microjoules per pulse per square centimeter produces sound at zero decibels, which is just barely in hearing range. To produce sound at 60 decibels, or the sound of normal conversation, requires 40 watts per square centimeter of radiation. ”That would kill you pretty fast,” Guy says. Producing an unpleasant sound, at about 120 decibels, would take 40 million W/cm2 of energy. One milliwatt per square centimeter is considered to be the safety threshold.
N.B. the 2018 JAMA report on the initial (21 cases) included sound that was "directional, intensely loud, and with pure and sustained tonality [...] high-pitched sound was reported by 16 (76%) although 2 (10%) noted a low-pitched sound [...] Owing to security concerns, further details of potential dosage cannot be provided." So it seems some gap remains on explaining how microwaves could have caused that "intensely loud" sound but not burns.
There is an (older) 2017 NYT article that has experts chiming in on the ultrasound hypothesis. The main issue with that seems to be that it's not clear how feasible the attack(s) would have been given how fast ultrasound pressure decays in air:
“Ultrasound cannot travel a long distance,” said Jun Qin, an acoustic engineer at Southern Illinois University. The further the sound goes, the weaker it gets. And, noted Dr. Garrett, humidity in a place like Havana would weaken it still more. [...]
An ultrasound-emitting device planted inside a building, on the other hand, might be close and powerful enough to cause harm to occupants. But even an interior wall would block its waves.
A smaller emitter placed even more closely, perhaps in someone’s pillow, might do the trick, said Dr. Qin. But it’s hard to believe such a device could escape attention. In theory, a building could be packed with small emitters; however, experts called it unlikely.
For what's worth it, AP released in 2018 a recording of what some of those involved in the incidents supposedly heard. But the US officials refused to confirm its authenticity and its relevance to the alleged attacks.
A recently declassified (via FOIA and with numerous blanked-out passages) 2018 report by a rather obscure JASON group said in its main findings that:
No plausible single source of energy (neither radio/microwaves nor sonic) can produce both the recorded audio/video signals, and the reported medical effects. [...] The recorded audio signal is, with high confidence, not produced by the nonlinear detection of high power radio frequency or ultrasound pulses. [...] We judge as highly unlikely the notion that pulsed RF mimics acoustic signals in both the brain (via the Frey effect) and in electronics (though RF interference/pickup). [...] It cannot be ruled out that the perceived sounds, while not harmful, are introduced by an adversary as deception so as to mask an entirely unrelated mode of causing illness in diplomatic personnel.
Much later in the JASON report, in an area where 99% of the text is otherwise blanked, it's mentioned that a possible source of the sound as heard might be a high-cycle concrete vibrator. If you don't know what that is or what it sounds like, see this vid.
Regarding ultrasound, while noting that it could be source of audible effects e.g. via intermodulation distortion, as source of physiological damage, the JASON report notes that
However, such lesions require direct contact with ultrasonic transducers.