The hardline cleric, whose extremist pact of Salafis had recently kidnapped Mohammad Abu Zahr in the Abra region of southern Lebanon, is shown with his companions hitting the man.
Assir had previously denied having attacked Abu Zahr.
The radical sheikh is on the run from the Lebanese military after his armed group launched a deadly attack on an army post last months, killing 20 soldiers and injuring a number of others.
The Takfiri cleric on Thursday called on his supporters in Lebanon to hold a demonstration on Friday.
He also accused the Lebanese army and resistance movement Hezbollah of cooperating and conspiring against him.
The audio message was Assir’s first public comment since he escaped from the southern Lebanese city of Sidon last month.
Also on Thursday, Lebanon’s Military Prosecutor Judge Saqr charged 37 people including Assir and his two sons over the recent clashes between the radical cleric’s gunmen and the army forces in near Sidon, the Lebanese newspaper Daily Star reported.
Saqr said the group was “forming an armed ring with the aim of carrying out crime against people and their property, undermining state authority and harming the state prestige.”
Sheikh Assir is a staunch supporter of Takfiri militants fighting the Syrian government. Takfiris dismiss most of Islamic sects as infidels.
Assir was unknown just two years ago, but he gained prominence as the spillover into Lebanon from the conflict in neighboring Syria has escalated. The cleric is believed to have hundreds of armed supporters in Sidon involved in the fighting.
Lebanon’s army has pledged to fight until it ‘finishes with’ the radical sheikh.