Best practice to have a 'category' fields where admin can choose wich voices the user can choose
You could use a Node Reference field where your 'Voices' are nodes that only administrators can make, that way you can restrict the maximum number of 'Voices' allowed in the Node Reference field settings, and you could turn 'Voices' off by un-publishing them, or if you need the boolean value for the Node output you could just add a Checkbox text field on the 'Voices' node and use the Custom Formatters module to render the Node Reference field out as you wish.