Getting people to do voices for their characters is one of the most insurmountable barriers when starting out as a DM. But, a great way to encourage this kind of roleplay is leading by example. Now doing voices can be a challenge, but it is not always as hard as doing a perfect cockney accent in a deep, gravelly voice. Here are some straightforward tips on how to create some unique voices that will take your roleplay, and perhaps your PCs', to the next level.
Accents are a great way to shake up your voices. It is the most common way to create distinct characters, and while doing different accents is far from the only way, it can be very effective. Whenever you do an accent the important part is to know your limits. Accents are hard, incredibly hard, so pick 1-3 that you're decent at and then modify them as you use them to make distinctions between different characters. Some of the next tips might help with changing those voices within an accent.
It may sound simple, but if your character speaks in a squeak or in an unusually deep voice, it is immediately obvious that it is a character speaking and not you. That's really the only goal of giving a character a voice, and so doing something as simple as changing the pitch of your natural voice goes a long way. It can also be combined with an accent to create further distinctions.
3. Stutters and lisps
Try not to use this in a mocking fashion, after all living with a stutter or a lisp can be difficult and it is important not to forget that. People who speak with a stutter or a lisp have unique voices, and we can use that uniqueness in our own voices when creating voices for our characters. Once again, it can be a small subtle thing, or it can be combined with other elements of your vocal range to create characters with truly fleshed out voices.
This one can be more difficult to combine with other vocal tricks, but even on their own, doing an impression of a character from movies or TV (or podcasts or radio) makes your character unique. The best part of trying impressions for characters is that, in general, your impression will be bad. I speak from experience, all of my impressions are bad. But in reality this is great, because if your impression is bad then your players won't see your character as the one you're imitating. Simply hold the character you're imitating in your head for everything you say in their voice, and your character will inevitably emerge with their own unique voice.
5. Tongue and Mouth Shape
A great and easy way to alter your voice is simply by changing the shape of your mouth. Speak with your lips barely moving, put your tongue against the roof of your mouth for everything you say, say everything out of the corner of your mouth while simultaneously keeping your tongue on the roof of your mouth. There are so many combinations and none of them require knowing accents, impressions or modulating your pitch.
Use this tip sparingly, but having a character that only laughs or whispers can be tons of fun. Obviously don't yell at your players, and make sure they can still hear you, but forcing them to ask your NPC to speak up or cool it is a fun roleplay technique. And, as with most of these, this can be combined with other tips to maximize its usefulness.
These tips can be applied to a character as easily as an NPC. The goal of doing a voice, as previously stated, is to ensure everyone knows when you're in character and when you're out of character. But also, voices are fun. They color the world and add depth to a character. Sometimes it's hard to convince players to use voices, sometimes it's hard to convince yourself, but as of yet I have never seen someone regret giving their characters that extra flavour.