The secret, it turns out, is frequency. The part of the sound that makes some people hear Yanny is higher frequency than that which makes some people hear Laurel.
As Lars Riecke, assistant professor of audition and cognitive neuroscience at Maastricht University, explained to The Verge "If you remove all the low frequencies, you hear yanny. If you remove the high frequencies, you hear laurel.
"If your... ears emphasise both the higher and lower frequencies, you can toggle between the two sounds."