When El Libro de buen amor was written in the mid-14th century, a time when the Spanish Language was still evolving into the modern standard with which we are familiar today. Certain graphemes were pronounced differently during this time period than they are today, causing the text to sound a bit different when read allowed.
For example, in a text from this time period, that is, a text in Old Spanish, we will find the that certain palatal clusters are in the affricate stage of evolution. This means that modern clusters like “c+e.i” were graphically represented as “ç + a, o, u”, “z” in this type of text and were pronounced as the cluster /ts/, which later evolved into the modern phoneme /s/ or /θ/ (for some varieties of Spanish). Additionally the modern clusters “c+e.i” evolved from the Old Spanish clusters “ç + a, o, u”, “z” which were pronounced as the palatal cluster /dz/, /ts/ and then later as /s/ in Modern Spanish (or /θ/ for some varieties). The grapheme “j” and the clusters “g + e,i” and “i” in Old Spanish would have been pronounced as the palatal cluster /ʃ/ or /ʒ/ which evolved to be pronounced as /x/ in Spanish. Finally the consonant cluster “ll” was pronounced as /ʎ/ in Old Spanish and evolved to be pronounced as the phoneme /j/ in Modern Spanish.
Below I have recorded myself reading a segment from the text to exemplify how the language might have sounded at the time.
Audio: Reading of section from El Libro de buen amor: