Audio of the tape was distributed to local media at Rosenberg's funeral, Juan Luis Font, the director of El Periodico newspaper, said.
Video of the statement was later posted on the Youtube website.
Fernando Barrillas, Colom's spokesman, issued a statement saying the government "categorically rejects any accusations made in tapes and statements being distributed to some news media".
"This reveals the intention of creating a political crisis around a case that should be investigated and processed by the courts," the statement posted on the government's website said.
Barrillas said the government would ask for international help in the murder investigation, insisting that Colom will be cleared once "the truth comes to light".