OCR - it's a translation from a scanned page into format understood by computers, usually for manipulation by some editing software...
technical info: Optical character recognition, usually abbreviated to OCR, is the mechanical or electronic conversion of scanned images of handwritten, typewritten or printed text into machine-encoded text.
I haven't used this 'Read Out Loud' function before....
Are these purchased / scanned books? Are they able to support this functionality? Have you changed / upgraded Adobe versions recently?
The book which is giving the "Warning! Empty page." audio message can be read aloud without problem using the FoxIt Reader. Which is another similar reader.
Though the electronic voice with this program didn't seem to be as natural sounding to my ears as the Adobe version, which is why I wanted to use the Adobe version.