Finally found sure Fix! First, make sure you haven't disabled Google Search. Once you're sure of that, go to Settings/Language&Input. Tap where it says "Current Keyboard: International Keyboard - Xperia Keyboard" and then a pop-up will appear with an option that says 'Choose Keyboards', tap on that, then you will finally find the 'Google Voice Typing' switch, turn it on. Now, when you go to Xperia Keyboard Options, the 'Google Voice Typing Key' switch will no longer be greyed out, so you can now switch it on to access voice input from the Keyboard.