Describing voices and voice mannerisms

Characterization

A (stereotypical) voice can convey a wide range of character traits to those around to hear it. From confidence levels to intelligence levels, from likely age to likely body type, and from emotional state to state of health. So while these elements may be too difficult to act out if you don't have the voice acting skills to back them up, they can be key points to use when describing the voice of a character.
Giving each character their own voice will also mean your world will feel more alive and prevent the sameness that builds up in many campaigns from doing so quicker.

In fantasy settings voices can be revealing in other manners too. Disguises, magical effects and other appearance changes could give players a clue as to what is going on. An orc with a squeaky voice could point at a gnome using an illusion spell. Describing two seemingly different characters' voices with the exact same, oddly specific adjectives could hint at one being a disguised form of the other. A voice described as mesmerizing and enthralling could hint at mind altering spells being cast. And so on.

Regardless of whether there are underlying hints or not, describing voices is just another way to make a character come to life more and to make them feel fleshed out. Their actions and words are part of this as well of course, but the difference between an insult thrown at you in anger, complete calm or with a good dose of sarcasm is significant. It reveals who the character behind the insult is, which in turn lets the players and characters know how to deal with this character. Anger could be exploited, complete calmness may require a more patient approach and sarcasm could be dealt with by throwing more sarcasm back. So besides fleshing out your characters, it can also be used as a tool to subtly steer the story a certain way assuming you roughly know how your players' characters will react to certain characters of your own making.

Focus

The easiest way to add these characterization elements is by focusing on just a few of them. Figure out what part of a character's speech stands out the most, which could be the tone, their emotions, their accent or melodic way of speaking, or even the words they choose and how they articulate them. If you're unsure about what elements you can pick or just want a fun exercise to experiment with, do the following: pick a character from a movie (or tv series/anime/etc.) and imagine how all the other characters within that movie say this character's name. For example, imagine how a wide range of characters in Harry Potter movies say "Harry Potter". From Snape to Dumbledore, from Ron to Dobby, and from Hagrid to Hermione. You'll soon notice many different details within their voices whether it's Snape's stressed syllables or pauses, Dumbledore's calm demeanor (wink wink), Hagrid's accent, or something else entirely.

These are the elements you can focus on as well, you could even give all of your characters voice traits of your favorite movie and tv characters. It'll make it easier to remember the voices of lesser characters too. "Talks like Snape with a deep voice" could make for a more helpful note to your future self than "talks with sarcasm, condescending pauses and has a deep voice", for example.

Speaking of notes, writing notes is always a good idea, but for voices it can be especially important. Even more so if you do a little voice acting as well, regardless of how bad or good you may be at it. A character may not show up for a while in between different play sessions, so their voice can be easily forgotten. Writing it down alongside their other details will help keep the character consistent.

Thematic descriptions

It's easy to fall into the habit of describing similar types of voices in similar ways, and there's nothing wrong with this. But when playing a longer campaign or one with many characters, having characters stand out can be very helpful to the players to make sure they remember them. While their behavior, look or other trait can do this too, sometimes a voice may be all you can work with, like a voice on a speaker in a spy thriller for example.

For some characters you can use regional, profession or physical traits to make their voice descriptions more personalized. For example, let's assume two characters, a miner and a sailor, have similar voices. The miner's voice could be described as "eroded by carbon fumes" while the sailor's voice could be described as "battered by salty ocean air".

Unfortunately this isn't always possible for every character, but for them you can change the phrasing of a description by using different words to reflect their nature. An emotionless voice could be described as "spiritless", especially for an otherwise spirited person, or as "devoid of life" for a character that deals with the undead, or as "flat" for a normally bouncy character.
This is obviously tricky to get used to doing during game-play or if you're just not that good at coming up with synonyms or specific words. This is completely fine and can be overcome by simply writing a few words in your notes to help you when the time comes to describe a voice. Like I said earlier, there's nothing wrong with sticking to describing voices in regular ways and then gradually expanding from this point.

Voice acting

Last, but certainly not least, is the voice acting side of gaming. It's an incredibly powerful tool, but unfortunately out of reach for many people beyond a few types of voices for a few characters. This is all you need if you're a player as you only control one character, but it becomes increasingly difficult for a game master as more and more characters come into play and in a stranger and stranger variety.
Voice acting itself isn't something I can write a guide on. For one because I'm definitely not the person to do so, I'm not a skilled voice actor. But I also believe it'll also be really difficult to convey voice acting skills, like changing the "position of your voice" within your body, in a written guide. If you are interested in improving your own voice acting, I highly recommend looking up tutorials on YouTube and similar sites. Videos can help a lot with visualizing what parts of your body you can use to make your voice sound different as well as hear the difference as the teachers act it out.

Having said that, there are still ways you can improve your own voice acting skills on a basic level. Those character notes I mentioned earlier can be a point of focus for your own voice acting. You may not be able to put a gravely texture in your voice or add a demonic growl or have a whispy voice of a tiny creature, but speaking with sarcasm, elongated vowels or ending every other phrase with "know what I'm sayin'?" is easy enough.
Simply trying to copy a voice you know can also go a long way to making a character have a more distinct voice compared to another. Even if you have little vocal control and/or are a terrible actor, changing the speed, intonations, pauses and accent with which you speak are relatively easy and can go a long way. Better still, you may do a terrible Morgan Freeman or Sir David Attenborough or Matthew McConaughey impression, but this could be of benefit to you as nobody will be able to tell you're trying to impersonate them. Your players may think you're trying to do an original voice instead. If not, your players may laugh a little, but, if Critical Role is anything to go by, it happens to the very best. In the end it's all about having fun anyway.