In the audio, a computer-generated voice says, "We are flying a plane into the Capitol Wednesday. Soleimani will be avenged."
The threat comes just a year after Soleimani was killed by a U.S. drone strike approved by President Donald Trump. Iranian leaders have in recent days said Trump himself would be dead "within a few days."
The audio threat heard Monday is not considered credible, but breaching air traffic frequencies is a crime and can endanger airplane safety as it interferes with pilot communications.
Pilots were told to be alert for any aircraft deviating from their standard flight patterns on Wednesday.