It's hard to sing a written song without knowing the rhythm or beat to it. Sometimes if I work at it I can come up with something but mostly I'll skip songs if they are really long.
Same with poems embedded in prose, very few authors can make a poem that's any good. Even Tolkien's stuff I skip sometimes, though I admit that they do add a lot of flavor to the story at times.
Another are long winded "prophecies", virtually all are rubbish at feeling authentic within the world. Sooooooooooo many bad fantasy works are saddled down with nonsense prophecy statements.
Best are short segments used to add a little color to the start of each chapter. Be it a quote, a snippet of song, or whatever.