I suspect the answer is mostly that they are not written in the voice of an in-universe character. The reason being is that most of the text is written in a third person limited voice, which is not the type of voice I would expect if it were being written as an account of historical events. 

Third person limited voice, as explained in depth in this blog post https://blog.reedsy.com/point-of-view/third-person-limited-omniscient is characterized as

> the author narrates the story from the close perspective of one character (at a time) to create the immediacy and intimacy of a first-person narrative, without being "trapped inside" a protagonist's head.

My reasoning why this literary choice seems to me to indicate it's not supposed to be an in-universe quotation is as follows:

1.  If the point of view character was the person relating the story, they would most likely speak in first person.
2.  If the books are a quote by a second party who was told the story by the point of view character, it seems to me unlikely they would not be able to write about the inner thoughts of the point of view character in such intimate detail.
3.  If the books are a quote by an omniscient or semi-omniscient character (which exist in the Cosmere), and therefore would have some idea about the point of view character's inner thoughts, they would probably not write in a perspective limited to a certain character's point of view.

The major exception being the epigraphs preceding each chapter in certain novels, such as the Mistborn novels, or the Stormlight Archive, which are written in a different voice entirely. They can be attributed to various different characters, as listed at https://coppermind.net/wiki/Category:Epigraphs

- Mistborn - Alendi
- Well of Ascension - Kwaan
- Hero of Ages - Sazed
- Way of Kings - Death Rattles (Moelach?) most likely recorded by King Taravangian, Hoid, Jasnah's notes from various sources
- Words of Radiance - Navani, quotes from Listener Songs, quotes from Words of Radiance (the in-universe book), Hoid, Taravangian's diagram
- Oathbringer - quotes from Oathbringer (the in-universe book), Endowment, an avatar of Autonomy, Harmony, quotes from the Urithiru gem archive, quotes from the in-universe book Mythica by Hessi, writes from the translation of the Eila Stele and from the The Way of Kings postscript