Due to the bug 17292753 mentioned here it is impossible to mute playback volume using device hardware buttons. The following methods are added since build 2.6.26 to mute

[[FPWCSApi2 getAudioManager] muteAudio];

and unmute audio playback

[[FPWCSApi2 getAudioManager] unmuteAudio];

It is necessary to subscribe to voluem change events to workarond the bug and allow to mute sound with hardware buttons (code)

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(volumeChanged:) name:@"AVSystemController_SystemVolumeDidChangeNotification" object:nil];

and mute audio when current volume value reaches the certain value, and unmute when this value is exceeded (code)

- (void)volumeChanged:(NSNotification *)notification
{
    float volume =
    [[[notification userInfo]
      objectForKey:@"AVSystemController_AudioVolumeNotificationParameter"]
     floatValue];
    
    currentVolume = volume;
    
    if (volume <= 0.0625) {
        [[FPWCSApi2 getAudioManager] muteAudio];
    } else {
        [[FPWCSApi2 getAudioManager] unmuteAudio];
    }
    
}