The Acoustics Laboratory at Helsinki University of Technology (HUT), the Sodankyla Geophysical Observatory (SCO) and the Finnish Meteorological Institute (FMI) all came together in an effort to study these sounds. This is the first study that managed to successfully catch the sounds attributed to the auroras.
Their cause is currently unknown and, for the time being, these sounds don’t seem to have a pattern as to when they occur. Obviously, because they are uncommon and have no determinable pattern, they are enormously difficult to study. Before now, the only evidence we've had that auroras make sound has been anecdotal (and, we all know how reliable that is).
It's also possible that the sound of the aurora is really a combination of physical, biological, and psychological phenomena.
Electrophonic hearing happens when nerves in the auditory system are stimulated by an external electromagnetic field. Under such conditions, subjects have reported clicking or popping sounds that happen in unison with light (the most notable example involves lightning, where witnesses 'hear' the lightning bolt before the thunder reaches them). In such conditions, it's the norm that other senses are affected. In the case of the aurora, hearing seems to be the only sense involved making it out-of-the-ordinary (then again, there is a very small subset of people who have reported odd smells when an aurora is out).
Another similar explanation uses a brush discharge to solve the mystery. Here, we theorize that the aurora actually extends to ground level causing ionization that is nod detectable to the human eye. This basically causes a buildup in static electricity that, in turn, creates little microscopic lightning bolts all over the place. Obviously, some observers might hear the static discharge allowing them to 'hear' the aurora.
It's also entirely possible that the sound is completely imagined, or a combination of 'imagined' sound with a real one (an example being: you hear a very light wind blowing in the trees, your heightened senses hears the leaves blowing, you don't feel the wind, and conclude the aurora is making a sound). This explanation fits best with the calmer or "sweeping" sounds that accompany the aurora simply because, if you're watching a wave of light swish back and forth, you're more likely to 'hear' a swishing sound (like ocean waves) that a defining crack.
On the video, you’ll hear a clapping noise, but the auroras are also (supposedly) responsible for sounds resembling swishing, a small animal scurrying through some dry leaves, or the crinkling of cellophane.
Have you ever heard an aurora?