I get this question a lot so I thought I would save myself some time and just make a page about it. With this easy to follow guide you will gain prestige and the adoration of millions of fans. Ok maybe not but you'll certainly be less annoying than the average Yahoo! user who cranks Nelly and puts the Mic up to the speakers.
STEP 1 - Collect Underpants
First you need to open the mixer. Down in your System Tray (lower right) you will see a little speaker. Double click that to bring up the Volume Control and click on Options -> Properties.
STEP 2 - ?
Once you get into the Properties you will see a small section labelled Adjust volume for: and three radio buttons. Select the one that says Recording. In the section labelled Show the following controls: make sure that Stereo Mix and Microphone are checked off. Now hit the OK button.
STEP 3 - Profit!
You should see a Volume Control window almost similar to the Playback one but with little check boxes at the bottom. The one you select is what Windows will put out to Ventrilo. Stereo Mix is the one you want. Be careful because ANY SOUND playing on your computer (chat on ventrilo, sounds from the game, your cutesy little AIM sounds that I want to choke to death and burn in a fire..etc) will be heard also! When you're done playing switch it back to Microphone. I keep this open to switch back and forth during raids.
Tips: Find a good level to play your music at usually at it's lowest setting. Don't piss ppl off with loud music. Play what people like, that's really impotant!