The UN says tests on a flight recorder found last week suggest that it is not from the plane that crashed in Rwanda, triggering the country's 1994 genocide.
Senior UN officials had "no knowledge" of the flight recorder
But spokesman Fred Eckhard said more tests were needed on the device which had been locked in a UN filing cabinet for a decade.
The spokesman said the recording lasted for about 30 minutes and some of the conversation was in French.
He said nothing heard so far proved it was linked to the downed plane.
He was speaking after experts from the International Civil Aviation Organisation listened to recordings from the "black box" in Washington.
Both Rwandan President Juvenal Habyarimana and Burundian President Cyprien Ntaryamira were killed in the attack.
The flight recorder was found after the French daily Le Monde published details of a police report which says that the UN had been sent the device a few months after the crash.
UN officials said its air safety experts locked the device away at that time after concluding it could not have been in an air crash because it was in "pristine condition".
Their findings were not reported to senior peacekeeping officials.
The French police report reproduced by Le Monde concludes that Rwanda's current President, Paul Kagame, gave direct orders for the rocket attack on Mr Habyarimana's plane.
Mr Kagame was then the head of the mainly-Tutsi rebel Rwandan Patriotic Front (RPF).
The genocide began soon after the plane was downed
The death of Mr Habyarimana triggered the mass killings in which some 800,000 people died, most of them Tutsis.
Rwanda has rejected the French report, describing it as "fantasy".
A former RPF officer, Captain Josue Abdul Ruzibiza, has told the BBC that he was ordered to provide security as missiles were fired at the plane carrying the presidents of Rwanda and Burundi.
Capt Ruzibiza said two missiles had been brought from Parliament House, where his battalion was being housed, and were fired as the plane carrying the two presidents came in to land.
The two men who actually fired the missiles, he added, are now senior officers in the Rwandan presidential guard and military intelligence.
Asked why he had kept quiet for so long, Captain Ruzibiza - now in exile - said he had lived in fear of his life.