As someone who played video games prior to my sight loss I can answer 19. Others may be more suitable to answer other questions than myself.

In a video game audio can be an important part of the game, both providing extra immersion and giving additional feedback. Audio can cover 360 degrees which is hard to do in video without a "radar display" type thing, which is often not appropriate to the game in question. It can also grab your attention more than visual signals, and helps you keep concentration on what you're doing without losing information you need (e.g. Mechwarrior 2's heat warnings).

In audio games for the blind the sound replaces the video, which makes for a different format. It is far more important to limit how much information is thrown out through the primary means of output, as opposed to a visual display which you can set up into different segments. Audio doesn't have this so when video output is not used you must output essential information in as brief a way as you can, whilst providing easy access (usually through hotkeys) to any other information which may be important or of use. It is not practical to use different parts of the sound output for different purposes so far, unlike video games which may have various indicators positioned around the screen in different areas.

The audio in a purely audio game usually includes such things as navigation information, sometimes in the form of a "sonar" such as in Terraformers and Audio Quake, or sometimes in the form of perhaps echoes when you're walking close to a wall as in Shades of Doom. Others yet with less restrictive environments such as outdoors have no such information in audio and output it through synthetic speech. This is for games from a first person perspective naturally, strategy games tend to feature speech more heavily whereas first person games almost all use 3D sound to help locate enemies. Video games may use 3D sound, but the enemies in audio games tend to make more noise and are often meant to be fairly easily recognisable by the sound they make.

I mentioned speech, that is important. Audio games often seem to use either prerecorded human speech, prerecorded synthesised speech, or increasingly common recently the use of synthetic speech directly through systems like Microsoft's SAPI speech API system (if you have Win XP or later you have SAPI 5 already installed).

Hope this is of some use.

edit:
Oh, some first person audio games have a "blip" type sound to indicate when you're aim is about right to hit a target (Tank Commander for example), this being roughly comparable to some video games which included a part of the display which lit up when you were aiming at the right spot to hit your intended target (e.g. the X-Wing series of space combat games).

Well, there are hobbyists as well as companies. I would consider RS Games and ACEGames hobbiests, while I see BSCGames as a real company as well as LWorks, Draconis, USAGames etc.

it is very true indeed that most if not all audio game makers are hobbiists, for one very simple reason. There are millions and millions of people who play mainstream games, an i don't know how many blind gamers out there, so the lack hereof really means that most adio game makers are rather limited, seeing as so few of blind people we have that actively play a lot of games do come across this. That's why i like this forum, to provide news on ags.
I agree with Ryan that you are a bit confused on audio games. audio games are games with no pictures at al. I've never played video games before so i don't know how to explain that... but, about do you make the sounds all by yourself, partially yes. most of the sounds are from sound libraries but there are some excellent home made sounds too.

Well Narada, as you see, audio in games is important. In particular, you might try Solaris Prime download I have. You can modify all the wave files with voice over into Dutch or German as I think German is a native language there and the game will be in Dutch. It is a simple purely audio game. That may get you an "A" from your teacher . The game is released as Creative Commons with commercial rights reserved. So for class room and school use this is fine. It is a small game so the under taking would not be to difficult as you are not writing code, you are just changing the wave files so that the phrases are in Dutch.

Narada,

1. How important is audio in a game?
2.    Why is audio so important in a game?
3.    Why do you use audio in a video game?

Unless a person is deaf, audio is very important in a game.
Background music sets the mood.
Incidental sounds give clues as to what is going on and where you are.
Speech tells the story and provides the feeling of really being in the game.

The other developers and gamers here have described how audio is important for gamers who are blind or have low vision.

  4.    Do you have different types of audio files?
  5.    Are there different categories of audio?

Yes.
WAV files are large analog files that often are used for speech or incidental sounds.
MIDI files are much smaller digital files that often are used for music.
There are other formats, too.

  6.    Do you make the audio files all by yourself?

Sometimes we make them. Sometimes we buy them.

  7.    What do you need to know before you make an audio file?

You need to know the differences among the audio file formats, which formats are larger, which work best for different kinds of sounds. You need to know what tools are available and how to use them.

  8.    Which programs do you use?
  9.    What do you think is the best program for making audio files?

At 7-128 Software we use Goldwave, SoundForge, and Finale.

10.    How do you put the audio in the game?

We use the Java programming language. It has pieces of code (called Application Programming Interfaces) that play audio files. Many of the other developers here use DirectX, which works with Basic, C++ and other languages in Windows.  Basically, you just write a line of code that says "play file X". You might say "play louder" or "make that voice sound like a girl"

11.    What is the best input?

What is best depends on all of the above.
Sometimes Marcia, our audio guru, who's a professional musician (and former rock and roll singer) composes music using Finale. This goes into a MIDI file. Sometimes she and I mix WAV files in Goldwave. For the video sound, we use a video camera with a mike, convert and edit the files using iMovie on a Mac and / or QuickTime Pro on a Windows PC and voila! The sound is in an MOV formatted video file. We also can have Cyndi, our professional actor, speak into a mike and record her dialog into a WAV file.

12.    What do you do first? Recording the audio or making the video game?

We do both at the same time. But we create all of our content: text, audio, video, and graphics at the same time as we are coding our games.

13.    How long does it take before an audio file is ready for use?

That depends on what's for dinner. If it's a big dinner, I fall asleep afterwards and it takes an extra day to do the audio. Seriously, it varies from an hour to several hours per file, depending on the above.

14.    How does audio cooperate with the video game?

Again, the audio adds to the game experience. It emphasizes the things we want emphasized.
Our playtesters who are blind demand "ear candy" the same as sighted folks demand "eye candy"
The idea is to make a game entertaining. Audio can provide interesting sounds, clues, and emphasis.

15.            Has the audio quality improved towards the last few years?

Yes it has. There are better tools. Computers can hold bigger files. CPUs can push sound faster.

16.    Do you think that a lot of people still play games if there