This might be hard to explain. I used to live in an apartment in New York whose windows faced the back of a church. One Sunday I woke very early, around 5:30 to 6 AM and with my eyes still closed I heard a choir singing, sort of off in the distance. I laid in bed, listening to this music, too lazy to get up right at that time, thinking of different things; this and that. All the time this choir was singing and getting louder. And then I really started to listen.
I have been to churches before and knew what hymns sounded like and I always considered them sort of bland, musically. But I had never heard a choir like this. The music seemed to follow no pattern, but it was complicated yet so beautiful. The scales seemed to rise and fall over and over again, with hundreds of men's and women's voices, basically singing "aahhh". I lay in bed just totally absorbed, by this time, and I remember thinking what an amazing choir this church has. I had lived there for 5 years and never heard a thing before this morning.
I got up and went to the window to look out and the music seemed to be coming from the sky somewhere to the left, way beyond the church. The sky had that just before sunrise look to it and I just stood there listening to these complicated scales sung by hundreds of voices. The music sounded not joyful, yet noble and regal and praising. It then started to recede and drifted away to silence. I had no explanation as to where this choir could be and how they could learn to sing that way. It wasn't until years later that I have read about angelic music and my guess is that I heard some too.