The recording features a 15-minute speech in a raspy, quavering voice purported to be that of 72-year-old Izzat Ibrahim al-Douri, who was Iraq's vice-president when US-led coalition forces invaded in 2003.
The voice in the recording, which could not be immediately confirmed to be that of Douri, praised "some groups of (insurgents) Ansar al-Sunna and, in addition to these, the heroes and knights of al-Qaeda and the Islamic State."
The so-called Islamic State (IS) has been fighting in Syria and Iraq and on June 29 proclaimed a "caliphate" straddling both countries and headed by Abu Bakr al-Baghdadi, who now calls himself Caliph Ibrahim.
Its militants spearheaded a devastating military offensive by a coalition of Sunni militant groups that swept through large swathes of northern and western Iraq.
"We give them a special salute with pride, appreciation and love," said the man in the recording, introduced by another voice as the great commander of the Baath party.
"A dear salute to their leaders, which issued a general amnesty on every one who betrayed himself, betrayed God, betrayed his country but then atoned."
He then went on to list several, sometimes obscure, Sunni militant groups believed to have rallied behind the so-called Islamic State for last month's offensive.
Saddam's regime was secular and Izzat al-Douri is believed to be the leader of Jaysh Rijal al-Tariq al-Naqshbandi (JRTN), or Naqshbandiya order, a group of Sufi inspiration long seen as a rival to militant groups such as IS.
The latest such message attributed to Saddam's red-haired right-hand man, one of the former regime's most recognizable figures, was released in January 2013.
After the December 2003 capture of Saddam Hussein, the wiry general nicknamed "Red Moustache" by some became the most senior figure, the King of Clubs, in the US army's infamous deck of cards of wanted Iraqis.
He was best known to Iraqis as "The Iceman" for his humble origins selling blocks of ice on the streets of Mosul, Iraq's second city and now a key militant hub.
Douri rose to become the number two official in the Revolution Command Council in Saddam's regime and was seen as one of the few men in the country who could stand up to the former dictator.
He was suffering from leukemia at the time of the invasion and thought by many to have died.
But his name resurfaced in unverified recordings linked to the JRTN, which appears to have evolved from a network of influential Sunnis akin to freemasonry into a fully-fledged armed militant group bent on undermining the Shia majority's government.
HH/HH