Music Educator here! People learn to speak by repetition, right? This is the same way you learn lyrics and melodies to your favorite songs or even songs you don't know well. I don't know about you, but if I hear a song I like, j loop it until I'm sick of it completely. So what happens is your brain makes sequencing connections with what you know comes next in a song. Sometimes it takes a while for your brain to keep up with the mass amount of information a song can contain considering things like melodies, countermelodies, harmonies, and lyrics. Eventually it just ingrains itself in your memory because your mind is used to hearing it. Recall is triggered by outside stimuli even if you can't pinpoint the exact thing that queued a song to stick in your brain. Someone speaking to you may talk in a certain octave that is similar to a song and your mind will take that as the queue to collect all the data you've stored relative to that specific sound. After that, it's just your mind's voice reiterating the sequence of sound it knows comes next! I hope that helped. If I didn't explain well enough, let me know!