Disclaimer: This guide is intended for those wishing to make an Internet voice demo for the sake of being scouted for online projects. Those who wish to make a demo for professional usage are encouraged to seek the advice of someone in the industry as the standards will differ.
I'm assuming you already know how to use Audacity or another audio editing program and save in mp3 format, as well as how to do basic volume leveling and music mixing. If you don't, there are plenty of tutorials around the Net. (There are so many technical aspects associated with making a voice demo that I won't go into them here.) If mixing is really a problem for you, contact me and I would be glad to mix your demo for you.
Here are just a few tips I've learned by listening to numerous demo reels, especially those of newcomers.
Avoid unnecessary talking in your demo. One of the instant signs of an amateur is someone who begins their demo with "Hi, I'm (insert screen name here) and this is my voice demo. I hope you like it!" It may seem "cute", but talking about this being your first demo or "It's not that good, but here it is" does not sound professional at all. It is best to just go straight into your voices. If you absolutely feel you need an introduction, a somewhat popular method is to enlist the help of a friend (often a deep-voiced male) to say your name at the beginning of your demo. But after that, it is highly advised to start off 'acting' - do not ramble on in your normal voice at the beginning or end of your demo.
Do not record your demo all in one take. It is too difficult to switch between voices right away, and one can almost ALWAYS tell when someone recorded one voice right after another. Record one, stop, listen to it, let yourself get into a different personality and then try the next. It is not uncommon to use clips recorded over period of weeks or even months.
Keep it concise. A demo should be a minute to a minute and a half long, depending on your range and experience level. Long enough to showcase your range of voices, but not so long that it drags on and bores the listener. You may think a two-minute demo helps people realize more of your acting potential, but in reality they'll probably just close out of it before it finishes. Shorter reels grab attention and leave people wanting more. Don't go TOO in-depth for each character - a sentence or two is enough. If you really want to go into an entire monologue, record that separately as an additional example to link from your site.
The first few seconds can make or break a demo for a producer hearing your voice for the first time. Start off with your strongest voice - preferrably something in a similar range as your natural voice and nothing too crazy or cartoony sounding. This is the voice that you want people to remember you by.
If you can't do a voice believably, don't attempt it. It's better to show off your strengths rather than trying to force out voices you're not good at or accents that sound unrealistic. If your elderly voice still sounds like a 16-year-old and your British accent still sounds like it was done by an American, wait until you've practiced more before including them.
Don't do BAD impressions. Obviously impressions cannot be exact, but if your attempt at imitating a popular character's voice really stinks and it shows, then avoid doing it. Do an original or more obscure character who has a similar voice type, or at least don't make it obvious who you're trying to imitate. Even if you absolutely love playing that character, one painful voice can give the listener an overall bad impression of your acting skills.
There are several ways you can go about finding lines to use for your demo. You can take lines from a cartoon or a manga, use lines you recorded for other people's projects online (if the movie or radioplay was their original work, it would be good to ask permission first) or write them yourself - assuming your scripts could pass for something professionally recorded. If you're not too good at writing believable dialogue, it would be best to stick with material already written or ask a friend to help you come up with some. However, never copy lines you heard in someone else's demo. There's a good chance that that material may have come from an original source, and besides, it's not all that unlikely that someone will recognize where they first heard it.
Avoid being too explicit. You never know if someone might be listening to your demo with their speakers turned all the way up and their parents in the next room - or maybe even downloading it at work! If you absolutely need some choice words to get one of your character's points across, it may be best to include a note on the page saying something like "Warning: Contains Profanity". Also, it isn't smart to include jokes of a racial nature or even stereotypical dialogue when doing certain accents. Just do a normal line in that accent - you never know if someone from that country may be listening to your mp3 and find it offensive. Anyone can act out a regional stereotype, so playing a unique character in that accent is much more impressive. How about an Australian gangster, or a desperate British housewife? Doing something unusual like that shows your ability to do the accent AND act out a believable character in it.
Have either a commercial demo, animation demo, or one of each. Don't mix promotional and cartoon voices in one file. Have one for your commercial/narration voices and a separate one for your "character" voices.
Having music and sound effects in your demo is highly encouraged. Use more than one background song, and make sure it fits the mood of your line (ex; if you have an energetic character line, don't have slow music playing in the background.) Make sure the music isn't so loud that it drowns out your voices (instrumental rather than vocal music is typically used as words in the background can be distracting). And due to all the copyright/filesharing/RIAA stuff lately, it would be a good idea not to use American popular music in your demo - even though it is generally considered 'fair use', it's better not to risk it. Try finding some anime or videogame music, or if you're really worried about copyrights, search for royalty-free music.
Remember that voice acting is ACTING. Include a range of emotions - bouncy, crying, yelling, etc - and personality types. Make the characters have memorable lines, not just say (for instance) "I'm Sonic the Hedgehog". Choose a more emotional line - perhaps an attack, or victory line. It doesn't matter if the listener knows what character it is. If they're looking for a Sonic voice, they'll be able to tell from that voice that you can do the part. Simply being able to alter your voice isn't enough - you want to prove that you can handle emotional dialogue.
It's great if you can do silly voices, but remember that most producers will be looking for something more natural. You can include a few of your cartoony voices but make sure to include a range of more "common" voice types that you'll need to use more often in productions.
Watch those technical flaws. You can have an awesome voice, but if you've got any distortion, pops, clicks, or static in your lines, it's a major drawback. Background noise is more noticeable than you think.
Make sure you have a decent place to host your demo. Don't use something which prohibits direct linking or something where bandwidth runs out frequently. This will prevent people from accessing it and they may eventually give up. Try a free host like putfile.com or even consider a reasonably priced paid hosting plan if you have a lot of samples.
Thanks to Haushinka from the VAC (VAC username Rina-chan) for taking the time to write this guide.