I don't understand why at one time I could not hear chords, and I can't understand why I can do it now.
One odd thing though, any song which I have learned to play without music in front of me, I can remember decades later, words, chords, tune, even inflections and fills.
Tunes which I have played from music, perhaps a hundred times, are gone as soon as the book closes. I could not tell you the key or the first chord ten minutes later.
Incidentally, I always take my song book to a gig. If I know it's there, I don't forget the words. I don't need to look at it. I don't even need to open it. It works, because in my mind, it's a failsafe plan B, so I have nothing to worry about.
Cheers
Dave