In Dungeons & Dragons, the dungeon master assumes the role of every non-player character. As a DM, when I must portray two NPCs at once, I often see the players grow confused about who is talking. Certainly if I were better at voices—and better at sticking to a voice—the confusion would lessen.
Short of voice acting class, I avoid the problem by contriving to have the players interact with one non-player character at a time.
In your favorite TV comedy, have you ever noticed how cast members with nothing to do leave the scene? Partly, this happens because actors hate standing in a scene with nothing to do, but moving extraneous characters offstage also focuses attention on the important ones. As a DM, you have a tougher job than a TV writer, because when you speak in character, the players may not always recognize your character. Find an excuse to trot out your NPCs one at a time, play their part, and then have them excuse themselves to go to the loo or to take cookies from the oven. (Many dark necromancers enjoy baking to unwind.) If you need characters to argue two points of view, let one convince the players, and then leave. Then have a second NPC meet to present an opposing point of view.
Now suppose you’re Mel Blanc and Jim Dale rolled into one awesome dungeon master. Great! Can I play at your table? Nonetheless, you still shouldn’t have more than one NPC onstage at once, you showoff.
As a dungeon master, you must work to offer each player as much time to play and interact as possible. That means that even if you can portray every member of the king’s council as they argue strategy, you should resist the temptation. Give the players a bigger role in the discussion by limiting yourself to a single NPC. If the players wanted to see a one-man show, they would have gone to the theater.
As you deploy your cast of characters, weigh the advantages of forcing the party to split up to meet NPCs separately. In “The Dungeon Master’s Guide 2 remakes the skill challenge,” I suggested splitting the party to force everyone to contribute to an interactive skill challenge. But even without a skill challenge, dividing the party always serves as a great way to encourage less-vocal party members to take the spotlight. In the dungeon, never split the party, but in the castle or guild hall, send them their separate ways.