How does that explain any of the sounds people recorded? I don't know how the gas fits into this part of the story.
Having personally done work with microwave oven magnetrons in open air, while recording video you can usually hear mains hum as the positive component of the AC line generates RF output. With more sophisticated RF sources and optics, you can probably make some pretty wild stuff.
The recording I saw shows a central peak around 7000 Hz with a bunch of side bands about 3000 Hz across. You can amplitude modulate just about any variety of carrier frequency, if an adversary was hoping to induce microwave auditory effect via a specific carrier frequency for tinnitus at 7000 Hz this would be a good method. If this is what was done, they were extremely reckless in how they did it, almost as if the only goal was to freak the embassy staff out.They likely would have maimed some of those peoples inner ears through excessive heating and they're the ones who noticed. Whatever it was, it definitely made the Americans flee Cuba soon after.