So be sure to get the latest update to 9.0 Beta. Yahoo it is about time you did something about voice in chat rooms for the many Vista users out there. I do not know what technology Yahoo is using for voice in chat rooms now. If you prefer to continue using 8.1.0.421 until 9.0 is out of the beta testing phase, then you will still need to install the True Speech Codec discussed in the paragraph below.

The next paragraph now only applies to those who want to keep using 8.1.0.421 and not the 9.0 Beta newest update.

This is because Windows Vista is does not include the True Speech Codec that Yahoo Messenger needs for voice in chat rooms and voice in conferences. Please note that this is different than PC to PC calling which is not affected by the missing codec. Good news is that you can install the true speech codec yourself. Please note that this fix works best for Vista Home Premium and not so well for Vista Home Basic. I am not sure about Vista Ultimate. But it cannot hurt to try the fix. Here is a link that has step by step instructions for downloading and installing this codec.
http://www.bigblueball.com/forums/yahoo-messenger-support/39794-how-obtain-voice-chat-windows-vista.html

Test your microphone – does it work in other programs (for instance, /Accessories/Entertainment then Sound Recorder)? If not, you may have a faulty microphone or it may be plugged in to the wrong opening on the pc.

However, I think what you want to do is use the sound system in the conference room (which is tied into your mixer).

You can do this easily by purchasing a phone that has a headphone jack and a microphone jack. Some inexpensive phones have this, and most high-end conference room phone will do this. Try Polycom, for example.

Be aware, however, that placement of the microphones and speakers is important. If not, you might get echos or the phone participants might not hear the speaker. This would be minimized with a good phone and well positioned equipment.