You'll just want to follow the outline here to open the right program. Start > All Programs > Accessories > Entertainment > Volume Control
from there you'll just want to go to Options > Properties as outlined and select Recording to see a list of available Recording Sources. Make Sure Wave is selected and then Click OK. From there the window on the right shows your list of Available sources. As shown click "Select" under Wave and you should be rock out on the yahoo voice chat without use of a mic. you'll probably have to adjust your levels to get a good sound if this is your first time doing this. and remember this will be where you go if you want to record with your microphone or use voice chat traditionally what its meant for in the future.