This is just like how people were wondering about record thirteen.
I believe that this is another story from one of the miners. One of them found the nether and recorded it with the eleventh record. Another found a skeleton dungeon and recoded it with the thirteenth record.
All the miners got lost on an expedition. They were each given orders to split up and a record to record their discoveries. I can't wait for more. That is my theory on these tapes.