Usage

Press the push-to-talk button and say voice command "DIRECTORY".

Select the address card you wish to tag.
Press the push-to-talk button and say "SAVE NAME".
Say the voice tag you want to save, e.g. "HOME".
Repeat the tag when requested.
There are similar commands for RADIO and CD, say e.g. "HELP RADIO" to hear about them.

This is how it would work for the lesser phone voice control only system:
Scroll down to the name you want to save a tag to.
Press the push-to-talk button on the steering wheel and say "STORE".
Say the voice tag you want to save, e.g. "HOME".
Repeat the tag when requested.
To delete a tag, press the push-to-talk button, say "DELETE NAME" and when requested say the voice tag and confirm with YES."

The voice tags only mode (so called speaker dependent) works a bit like the latter expect you don't have to say "STORE" or "DELETE NAME" at all - you just click the pust-to-talk button over directory entries and it will store/delete depending on if a voice tag is there or not already.

Overview

There are 3 types of speech dialogue available depending on what is fitted to your car.

1) Phone voice control. The language can be selected with VAG COM. but speech control is limited to control of the phone functions.

2) "Speaker dependent" voice control without dialogue. Very limited mode that is language independent in that it only recognizes user-programmed voice tags and answers with beeps.

Only exception is some U.S. Audis that have the phone speech control disabled, but it too can be enabled with VAG COM (it is just flipping a bit on). All MMI phone-prep cars come with both of the above in hardware, either (or neither in some cases, e.g. if the third option below is installed) is enabled via software configuration.

The above voice control options are also available on many non-MMI-based Audis that have a radio and a phone-prep/Bluetooth module.

Third kind is available when the car is fitted with speech dialogue system, it requires full MMI

3) Full Speech dialogue system with controls for phone, radio, CD, navigation, directory. Correct language module must be installed in car, only one language fits inside the module software at a time (upgrade CD can be used to install new language). This is a module inside the radio module in trunk, very easy to retrofit too (assuming your car already has the phone module so you have the microphone).


Coding
If your Audi has a Bluetooth/phone-prep module, you can simply adjust the first and second types of voice control with VAG COM:

[ Telephone - 77 ]

[Coding - 07] -> Long Coding Helper -> Chang e 2nd to last digit

Select: One of the following

0 - Disabled

2 - English UK

9 - Voice Tags only

There are a few different languages built in too (e.g. numbers 3-8), the VAG COM helper tip will display a list.

If your car has or you have retrofitted the third voice control type, the full speech dialogue system, the VAG COM coding is a bit different. Because the language is fixed, only one language is loaded in the module at a time (other languages can be loaded from Audi maintenance shop MMI upgrade CD), you only have to enable the module in coding. First, make sure you have set the above phone voice control value to disabled (that is, 0).

Then check the CAN gateway (module 19) coding checkboxes in VAG COM, the speech/voice control check box must be enabled there. If your VAG COM error report shows a component protection error for the speech dialogue system module (and pressing the voice control button in the steering wheel only results in immediate CANCEL), your dealer must remove theft protection from the module before it can be used.

If your car has the first or second voice control type and you want to install the full speech dialogue system, the third type, and your car has full Audi MMI, all you need to source is the voice control module. It does not need any cabling. Kufatec.de will happily provide you a used one and your Audi dealer spare parts department is a source for new ones. Just make sure you ask for the right language one, since unlike the phone voice control which language can be configured with VAG COM, this one will only house one language at a time (and must be loaded with new software to change language).

Installation

whether you need to remove the component rack behind a cover on the left side of the trunk depends on how your cars equipment has been fitted, some have the radio module on the outside of the rack and needs only to drop the NAV DVD down to make room and insert the speech dialogue system module inside the radio module. you will need to prise open the cover n top of the radio to insert it. others are on the insert and removing the whole rack to get access is required. put everything back in place - and the instal is completethat's it! If your car does not already have any of the Audi factory telephony options (and thus the related microphone), you need to retrofit that too

Check this links for more information (links updated 20.2.2010):

http://a6retrofit.tripod.com/articles/a6phoneprep.html (retrofit Bluetooth and voice control in an A6, retrofit for A4 B8/A5 with navi/MMI are pretty much the same)
http://www.a5oc.com/forums/showthread.php/voice-control-question-4339/index.html (voice control thread on A5OC.com with comments by myself that parts of this article are based on)
http://a6retrofit.tripod.com/articles/a5vagcodes.html (Audi A4 B8/A5 VAG COM codes, including codes for adjusting voice control)

UPDATE 25.1.2009:

Here are links on how to change the SDS module language (basically how to use an MMI update CD to change the SDS module software):

How to change SDS language
http://audiforum.us/mmi/10403-how-change-sds-language.html

How can I change the language of speech the SDS ?
http://audiforum.us/mmi/6394-how-can-i-change-language-speech-sds.html