Another poster mentioned this - hearing it differently from different sources. Probably to due with the frequency response of the source and/or speakers. My kids were doing this the other day. My son could hear both. My daughters hears the higher pitched one. Then when my son explained the lower voice one, one daughter could hear both, and the other daughter could only hear the low-pitched one! (Initially only hearing the high pitched one!!??)
Reminds me of those visual tricks with the various things - do you see the beautiful gal or the ugly hag? I think there are some things that you can do to confuse your sense of touch as well - can’t recall what they are now.
I agree with your post most of all. It is true that the sound packet can be broken down into different harmonics, our brains also Hear what we hear by identification. On our TV we tried the sound repeatedly with the base and treble set at different points and my wife kept hearing the Y sound while I heard the L sound.
I am told that the recognition of the Y and the L are similar in our brains and we can “flip” this illusion in the same way that optical illusions can flip — for example the old hag and the young lady.