Sadly, I agree that our brains have the ability to interpret what we see and hear and feel. I don’t like it, because it means we cannot always believe our lying eyes.
In this case, I go with those who think this sound is illusory in the sense that it is close in many people’s experience to sounding like either of the two choices. So our brains pick one or the other. I never heard the Y sound, but my wife hears both at different times and so I have to believe her mind is having trouble catching the same sound each time.
The dictionary website person said they had hired 100’s (?) of Broadway performers to do the annunciation of the words. This one was done by one of the original performers in “Cats”. He has no explanation for why people are hearing “Yanny”.
And seeing as “yanny” isn't a word - he decided to make it one, and put it in his dictionary. He put it's meaning down as something about having a difference of opinion on the meaning of social internet posts (or something like that)!