It simply wouldn't be worth the time and trouble to manage such a scheme and adjudicate submissions. Besides one needs the voice actors to be available for script changes later. If users could create voice over text essentially at no cost, for small parts, there's no reason why your local office staff couldn't do the same - professional actors are chosen for a reason - i.e. you can get the results you want more easily. Early games often didn't use actors, because of the expense - but you can tell :)