A list of evidence potentially in play at the hearing is required to be submitted by prosecutors. The potential for jailhouse evidence had not been previously reported, The Denver Post said Friday.
The nature of the recording is not known.
Also on the list are spent shell casings from a local shooting range where prosecutors suggest Holmes went for target practice before he allegedly shot up a movie theater in 2012, killing 12.
Prosecutors also included photos Holmes took of himself with the weapons used in the attack, and a Batman mask. The movie "The Dark Knight Rises" was playing at the time of the massacre and Holmes had dyed his hair orange like the villain The Joker, the Post said.