The key to a good NPC voice is not to try to speak in an unnatural way. RPGs are a game of imagination and your players should supply the character's true voice in their imagination. It is your job to suggest it. So, if you don't have a deep voice, speaking a little lower and trying to use more breath, so you're a little louder, may be enough to identify what is different about the NPC's voice. You're a storyteller, not an impression artist. If you listen to a book-on-CD narrated by Patrick Stewart, he's not going to go into a falsetto most likely for female characters. Instead, he's going to suggest more feminine characters, by using touches that suggest the character, rather than imitate them.