If you don't mind using GUI Scripting (you must enable access for assistive devices in the Accessibility System Preference pane), the following script should toggle the Dictation switch between On and Off.
tell application "System Preferences"
reveal pane id "com.apple.preference.speech"
-- activate -- only if you wish to make the Dictation & Speech window visible
tell application "System Events"
tell process "System Preferences"
tell window "Dictation & Speech"
tell tab group 1
click radio button "Dictation"
tell radio group 1
if value of radio button "On" is 0 then
click radio button "On"
click radio button "Off"
if sheet 1 exists then
tell sheet 1
click button "Enable Dictation"
repeat while sheet 1 exists
quit -- optional
Hope it can help.
Message was edited by: Pierre L.