This is an interesting theory. Are they really mixing the music for todays gear? Or are they doing something like BBE or a type of 3D sound to the mix?
I have long pondered that there may be something to this. Like running the entire mix through an Autotune? Or throwing out Musical information that is not needed like the MP3 Codec does to individual tracks and then mixing them together so they don't sound so congested. Maybe making the waveform having less harmful sonics that can show up in the mix. Or EQ in a way that sounds better for modern mass market speakers and staying away from troublesome notes that the mass of inexpensive speaker / drivers have a hard time reproducing such as those in the crossover pass band. Etc.
Kind of like mixing your CD you made in a way that has DSP settings and EQ you need to match your cabin space with time alignment already set for your listening position.
Occam's razor would suggest its just in your head. Or you have better gear now then you did back then. Speakers have improved so much today? Its hard to argue that listening to the recording today may sound better for the simple fact you have better gear all around.
I think psychoacoustics may play a factor. Music and emotions do seem to have a tie to each other as the M
ayo clinic stated:
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I am not one to quote string theory to be smart, but I do think that there may be a tie to EM fields and how your brain and body may interpret the experience, specifically on its memory and its "Coloring" of the event in your musical memory.
You also have to think about your endorphin levels and other chemical factors as well.
Its well known the your teenage love will be one of the strongest memory in your love life. Helen / fisher talked about this to some extent and did some research.
Not sure of the validity.
I would think the oxytocin decreases with age however I have been corrected on that and so far it seems that the research points out that it increases with age. So maybe that is the reason it sounds better now possibly.
But in all likelihood? May just be that you have a better setup now. It's a toss up without real data. And just an opinion otherwise. Not a factual event in time as there is no data to confirm it.