Lusaka - The trial of a Zambian musician charged with conduct likely to breach the peace over his song which accuses President Edgar Lungu of drinking too much, was on Wednesday postponed to July 13.Chama Fumba was arrested early this month over the song which some government officials allege was defamatory to the head of state because it accused Lungu of being incompetent.In an interview with AFP on Tuesday, Fumba - whose stage name is Pilato - denied allegations that his song was either defamatory or likely to have disturbed the peace."I don't think that [it] is defamatory in any way. I don't think it is causing any breach to our beloved peace," said Fumba. "I don't think it's a controversial song."But he warned that freedom of speech was under threat in Zambia."As it is now, I can assure you that it's under very serious threat," said the musician. "I feel that the powerful who happen to be the politicians who originally were supposed to be our servants have become our masters."Opposition politicians"They don't want us to exchange ideas through expression, they don't want us to talk about them."The song's lyrics tell the story of a character named Lungu who grows up in a slum and becomes president.The Lungu in the song is described as carrying a suitcase filled with bottles of Jameson Irish whiskey and having no idea how to govern.President Lungu, 58, who took office in January, is a former defence minister.He suffers from recurring achalasia, a condition caused by narrowing of the oesophagus, and has been regularly hospitalised for treatment.His election victory followed the death of predecessor Michael Sata.Fumba's trial failed to commence on Wednesday due to the non-availability of state witnesses.If convicted, Fumba, 31, faces a maximum sentence of six months in prison or a fine.The case has attracted the interest of opposition politicians, civil society organisations as well as US and German diplomats in Zambia, some of whom were in court on Wednesday.