Since then, at least 22 Americans have been "medically confirmed" as affected, but by what is not yet clear.
Cuba has denied involvement in any attacks. The U.S. has not assigned any blame.
In the recording, pulses seem to come in varying lengths with sustained periods that are minutes long.
While some of the affected Americans reported hearing the sounds--often coming from isolated spots in a room--not all those who've been sickened reported hearing sounds.
RELATED: US embassy in Cuba
US embassy in Cuba
US embassy in Cuba
People walk past the U.S. Embassy in Havana, Cuba, June 19, 2017. REUTERS/Alexandre Meneghini
A tour bus of Transgaviota drives past the U.S. embassy in Havana, Cuba June 13, 2017. Picture taken June 13, 2017. REUTERS/Stringer
An exterior view of the U.S. Embassy is seen in Havana, Cuba, June 19, 2017. REUTERS/Alexandre Meneghini
People wait in line to enter the U.S. Embassy in Havana, Cuba, April 20, 2017. Picture taken April 20, 2017. REUTERS/Alexandre Meneghini
People wait to enter the U.S. Embassy in Havana, Cuba, April 20, 2017. Picture taken April 20, 2017. REUTERS/Alexandre Meneghini
A vintage car passes by in front of the U.S. Embassy in Havana, Cuba, January 12, 2017. REUTERS/Alexandre Meneghini
Cuban flags fly near U.S flag beside the U.S embassy in Havana December 31, 2015. REUTERS/Enrique de la Osa
A man lowers the Cuban flag while standing amidst flagposts installed outside the U.S. embassy in Havana, December 18, 2015. REUTERS/Alexandre Meneghini
Flagposts installed outside the U.S. embassy cast their shadows on the sidewalk of the seafront Malecon in Havana, December 18, 2015. REUTERS/Alexandre Meneghini
Tourists pass by the U.S. Embassy in Havana, February 18, 2016. Picture taken February 18, 2016. REUTERS/Alexandre Meneghini
Competitors run past the U.S. embassy during the Marabana marathon in Havana, November 15, 2015. In the year since detente, more Americans are visiting Cuba, and more Cubans are trying to reach the U.S., concerned that special treatment for Cubans may end. While foreigners are in a frenzy, most Cubans report little change. Although they have guaranteed education and healthcare and minimal fear of violent crime, their wages are poor and economic opportunities limited. Picture taken November 15, 2015. REUTERS/Alexandre Meneghini
The Cuban flag flies at half staff in recognition of the death of Fidel Castro, the long time leader of Cuba, at the Cuban Embassy in Washington, U.S., November 28, 2016. REUTERS/Joshua Roberts
Discover More Like This
BACK TO SLIDE
The AP reports it is also unclear whether all those who heard the sound heard the same sound.