The majority of Earth's natural radio emissions audible with ground-based radio receivers occur in the extremely-low-frequency and very-low-frequency (ELF/VLF) radio spectrum - specifically, at AUDIO frequencies between approximately 100 to 10,000 cycles-per second (0.1--10 kHz). Unlike sound waves which are vibrations of air molecules that our ears are sensitive to, natural radio waves are vibrations of electric and magnetic energy (radio waves) which - though occurring at the same frequencies as sound - cannot be listened to without a fairly simple radio receiver to convert the natural radio signals directly into sound.
Whistlers are magnificent sounding bursts of ELF/VLF radio energy initiated by lightning strikes which "fall" in pitch. A whistler, as heard in the audio output from a VLF "whistler receiver," generally falls lower in pitch, from as high as the middle-to-upper frequency range of our hearing downward to a low pitch of a couple hundred cycles-per-second (Hz). Measured in frequency terms, a whistler can begin at over 10,000 Hz and fall to less than 200 Hz, though the majority are heard from 6,000 down to 500 Hz. Whistlers can tell scientists a great deal of the space environment between the Sun and the Earth and also about Earth's magnetosphere.
The causes of whistlers are generally well known today though not yet completely understood. What is clear is that whistlers owe their existence to lightning storms. Lightning stroke energy happens at all electromagnetic frequencies simultaneously--that is, from "DC to Light." Indeed, the Earth is literally bathed in lightning-stroke radio energy from an estimated 1,500 to 2,000 lightning storms in progress at any given time, triggering over a million lightning strikes daily. The total energy output of lightning storms far exceeds the combined power output of all man-made radio signals and electric power generated from power plants.
Whistlers also owe their existence to Earth's magnetic field (magnetosphere), which surrounds the planet like an enormous glove, and also to the Sun. Streaming from the Sun is the Solar Wind, which consists of energy and charged particles, called ions. And so, the combination of the Sun's Solar Wind, the Earth's magnnetosphere surrounding the entire Planet, and lightning storms all interact to create the intriguing sounds and great varieties of whistlers. (http://www.auroralchorus.com/vlfstory.htm)
This video gives a good explanation of McGreever's work along with sound clips: