By now, you're firmly in camp Yanny or Laurel, your marriage is in shambles, and up is down. This latest viral illusion was of the auditory variety, thus sparing you from a long angry diatribe about white balance and monitor calibration. I'll admit that I first heard Yanny and it immediately shifted to Laurel, mostly ruining the experience.

If you've spent any time mixing or mastering music, you know to aim for a solid middle-ground mix that sounds good on most stereos. This means making concessions that make the mix sound objectively worse on expensive audiophile level gear, but better overall across more accessible equipment. Certain mix combinations bury instruments or vocals entirely because of how the audio is processed and the speakers' frequency response.

Mass-produced speakers cannot replicate every single audible frequency, particularly the tiny ones in your smartphone. These smaller speakers generally sound more shrill as there isn't enough volume to produce audible low frequencies.

The Y/L illusion results from stripping the lower frequencies from the word Laurel leaving us with something that sounds completely different. The recording is consistent with other online English language pronunciation guide videos. As Yanny isn't a word, it seems pretty clear that this illusion is merely coincidental and isn't two distinct superimposed recordings.

Another possible explanation is the compression of the audio via however you are hearing it (twitter/youtube etc). Slower connections results in the quality dropping automatically to avoid buffering, and this compression mostly removes the lowest frequencies. Just as the tiny speakers cant replicate low frequencies, the compression can also strip out the low end, leaving us again to hear Yanny. 

Lastly, the most fun theory can be extrapolated as survival instinct. It becomes impossible for me to hear Yanny after I have deduced that the voice is in fact saying Laurel. My brain will fill in the gaps (the low notes) from memory. Even if I lose the low end, or listen through a lil' tiny speaker on a sloooow connection, I will still hear Laurel. Try as I might, I can no longer hear Yanny. This phenomenon utilizes the same part of our brain which helps us sync up what we hear and what we see, as this information arrives in our brain at completely different times (speed of light vs speed of sound). This skill keeps us evolved humans from feeling perpetually disoriented. All of this is good because if Yanny is anything like Yanni I don't want to hear it anyway.