Well, although I agree with GiGs that you don't have to, I'll note that "don't have to" isn't the same as "shouldn't". For me, successful DMing is largely about building immersion in the world for my players. And I mostly do that with maps and carefully worded descriptions I read out, as I'm a visual/text person. In the end, it's a personal style. I have less than no acting ability, so I rarely try to get "in character" as a DM as it's not likely to work well. But I've had a few characters I've played where I did adopt a particular speaking style for (more word choice than accent; I often script at least some of the major NPCs in advance with ways they'd respond to specific questions). But I've known both players and DMs who did "characters" (accent, attitude and speaking style) more often, with varying degrees of success. If a voice synthesizer helps with the characterization of an NPC (or a PC for a player), it seems worth at least trying. Just keep an open mind (and ear) to the opinions of the table. Sometimes you can't tell when you're failing, and the players may not wish to tell you, so make it clear that they should speak up if something falls flat, or worse, gets in the way of understanding the situation.