Wow, I thought I was the only one. For me, this only happens when I'm really tired and laying in bed for a while, maybe it's a form of Wake Induced Lucid Dreaming (WILD). Sometimes I hear songs I've never heard before and I scramble to write down the melody/lyrics. Other times it'll be mashed up memories mixed with other things. It's all over the place and out of order. I'm kind of in a sleepy state while it happens so I'm not too concerned of it being yet another diagnosable illness.