I don't know anything more about this case than what I read in the article, but the point I want to make is a general one about the challenges of performing audio and video forensics on evidence that is recovered by a crime scene investigation.
... an address book that contained references to "choking," "chasing" and "sexual desires." Other evidence includes duct tape with hair; a video camera; and a videotape of Spicer being sexually assaulted and beaten.
Sanders said more than 20 videotapes and some audio tapes, including at least one 90-minute tape, were found in the truck Davis, 41, and Riley, 39, crashed in Barton County, near Lamar, Mo., last week. They were found by Barton County Sheriff's officers. Police also found videotapes where Davis worked. The tapes are similar to the one found at Davis' apartment and show Spicer and Ricci being abused.
When a technical surveillance unit (TSU) collects audio and/or video, by and large, the forensic examiner can reasonably expect that any media received for examination will have been recorded using standard equipment and procedures, as well as the fact that most, if not all, of it will be relevant.
Note: This does vary, of course, due to skills, training, equipment maintenance, equipment budgets, and operational constraints (e.g. time and opportunity). Just because it was recorded by a tech surveillance unit doesn't mean that it will be perfect; however, the odds that it will be useable are improved substantially.
Not so with recovered evidence - who knows when the last time the tapes in the answering machine found at the crime scene were changed? Probably not in the last two years, Murphy's Law says! Was an external or internal mic used on the video camcorder? Yeah, right. Was the lens clean on the camera? Was the right lens used for the lighting and distance involved? Was the audio and/or video over-compressed? Were the batteries freshly charged? Did he (i.e. the suspect) try to perserve the evidence or destroy it? Did the "black box" come through the crash unscathed? The list goes on and on...
The long and short of it is that recovered audio/video evidence is like a grab bag at a tourist trap - you never know what you are going to get, but you can pretty much expect that it won't thrill you. And in cases such as (videotaped) sexual assault and murder, it can be as, or even more, grim than what the classic 'wet' forensic disciplines deal with (e.g. blood, semen, autopsies). In the context of this post, recovered evidence is the most challenging type that an audio/video examiner can receive, by far.