If you have the "Minimize to System Tray" option checked, the "VoiceAttack" entry will be removed from the taskbar, and will not show up in certain dropdowns, like the one for the "Set Audio Level" action, as it cannot be used for targeting.
You could try targeting the process, rather than a window title, by prefixing it with the "~" character, E.G.
Toggle mute for application [~VoiceAttack] audio