A shocking audio recording of the moment an injured Afghan insurgent is shot by a Royal Marine sergeant was released by a military court yesterday.

The pistol shot to the helpless man's chest can be clearly heard – followed by the marine telling the man: "Shuffle off this mortal coil, you cunt," and instructing his fellow marines: "Obviously this doesn't go anywhere, fellas … I've just broken the Geneva convention."

Three marines – who can be identified only as A, B and C – are accused of murdering the man, who had been badly wounded in a helicopter attack, in Helmand in September 2011.

A is alleged to have fired the fatal shot, while B and C are accused of helping and encouraging him. All three deny murder. The board hearing the court martial retired yesterday to consider its verdicts.

The incident was captured on a head camera worn by Marine B. The court refused to release the video after the government argued it would be a gift to terrorist recruiters and could prompt "lone wolves" to carry out revenge attacks against British soldiers or civilians.

But the court agreed that the audio recording should be released. It also said that a few still images taken from the video could be published – but not ones showing the insurgent or the moment he was shot.

During the first audio clip – identified as video clip four during the trial – the marines are roughly dragging the injured insurgent across a field and aiming abuse and laughter at him.

Marine A can be heard allegedly ordering his colleagues to get the man out of sight of a British observation balloon. Marine C is heard suggesting he shoot him in the head. "I'll put one in his head, if you want." But A replies: "That'll be too fucking obvious."

The prosecution alleges that clip five shows that the marines waited until the British helicopter that had injured the man had left the scene. Marine C is heard saying: "Maybe we should pump one in his heart." After the shot is fired, Marine B says that if the shot was heard by anyone they should say it was a warning shot. Speaking on the radio, Marine A says: "He's, er, fully dead now."

The still images give an impression of the area the men were working in. They show the cornfield the man was shot in and the mound he was dragged on to at the side of the field. They also give an idea of how heavily armed and well-equipped the marines were.

Marine A accepts he shot the man in the chest but claims the insurgent was already dead and he was firing into a corpse out of frustration. Marines B and C deny they encouraged or helped A. Marine B conceded he was prepared to cover up what had happened to help a fellow marine. Marine C claimed his remarks about shooting the man were bravado.

The court earlier released a transcript of the video produced by the prosecution.