Controller built-in vs external DAC
Well, if by voice you actually mean human speech or singing, it appears you don't need fast sampling rate to satisfy Nyquist (which means that built-in DAC is way to go), another strong suit of built-in DAC is fact that you don't have to implement any serial communication with external IC, you could just use your MCU instruction set to control your DAC. The downside for built in circuit is fact that it's only 12 bit wide which means that you won't be able to achieve even audio cd standard (16-bit,44100Hz). 
My advice would be to test it - code your way around built-in 12-bit DAC, listen to how it sounds. Then try using 16 or 24 bit external DAC and see if difference between both results is significant enough to go with external DAC