I emphasized the audio and video related items with a bold font in the quotation.
"Some 30 kilograms of explosives, dozens of guns, a suicide bombers vest, and a videotaped last will and testament were confiscated in raids on three apartments being rented out by the two suspects arrested in October.
The video tape shows the two men asking God for forgiveness for the sacrifice they were about to make. The two suspects are also shown making bombs, including one planted in a lemon and another planted in a tennis ball.
Police also found face masks worn by two of the suspects in the videotape and hair samples from those face masks believed to belong to one of the suspects. However, Bosnian forensics teams do not have the technical capability to analyze these samples.
The Bosnian authorities sent the videotape, a video camera, and other evidence to the US Federal Bureau of Investigation (FBI), which sources said were analyzing the samples.
The FBI’s forensic tests had shown that the video camera was the same one used to record the confiscated video tape containing the bombing-making evidence and the last will and testament. The hair analysis is expected later this month. The voice samples from the videotape could not be verified, the Bosnian police source said.
The source also said that comparing the number of weapons found in the apartment rented by Bektasevic and Abdulkadir with the number of weapons seen on the videotape, it was clear that weapons and explosives were still unaccounted for."
Having both the original tape and the recorder allowed the examiners to match them up. This can be done by different means, depending on the type of recorder, including using ferrofluid and a microscope to look at the magnetic tape (i.e. 'rusty plastic') to see the the patterns left by the head while erasing/writing.
The examiners reportedly weren't able to succesfully perform speaker ID (identification). Typically, this can be due to several factors, including: muffling of the speech (in this case by the masks they were wearing), interfering noise, or insufficient speech quality (e.g. level, dynamic range, frequency response, bandwidth).
This case also highlites how forensic units around the world can be of assistance to one another, as well as how proper equipment and techniques can yield valuable clues and evidence.