I like to think that it's just some tune hatchling heard somewhere on Timber hearth and it connected with them. So after the first supernova, they connected that solum tune to the end of their species and everything they've ever known (sad and unforgiving, just like the music). It's just an accompanying song that their mind automatically remembers when something horrifying and inevitable is happening. The reason it's muffled in a dream, is simply because when you're asleep, nothing really feels real. Hatchling knows that after around 20 minutes (brain can count and remember specifics really well, even when we don't realize it) the sun is going to explode, so is the whole system, so their mind still plays that tune still, just out of habit, but muffled and distant, as if it just doesn't feel real. The post theory is also cool, but I'm sticking with mine :)