I am looking for a children’s picture book from the late 1970s, early 1980s. The book was on natural/beige paper with brown line drawings. The story was a fantasy where a young boy lives in a small cottage. Every night, flying creatures (birds? harpy-like things?) fly over his home and sing him to sleep. Possibly as the sun sets. They sing the same song every night. The lyrics were along the lines of “Remember my friend of the song of your heart. Remember my friend, for the rest of your life. Love conquers all, for it never grows dim. Love conquers all, for all time”. The book came with a cassette tape and my siblings and I could probably hum the song to this day. One night, the bird things don’t come and sing. The boy gets worried. He hears knocking at his door or window. It is a talking animal or non-human of some sort. He hears that the local monster thing that lives in a cave or mountain through the woods has stolen the bird things and plans to eat them and/or make them sing only for him. The boy says, “We must get them back. We must!”. The boy climbs on the back of a horse or four-legged animal and they race through the woods to the cave/mountain. The boy sneaks into the cave where the bird things are in cages. He releases them somehow and they escape. Not sure what happens to the monster thing. The book ends as the bird things once again fly over his house and sing him to sleep. I want to say that the monster thing is a Gorgon but I’ve maybe conflated Greek myths later on with this early fantasy children’s book. Or, it is a really well known adaptation and this will be an easy solve.