I know, right? :o) Here are some tactics to try:

1. Incoming sound: Click the Speaker icon in your system tray (on the bottom-right corner of your screen) one time and drag-and-drop the volume bar to the top if you’ve been having trouble hearing. Lower it if sound has been too loud. Make sure "Mute all" is not checked under “Volume”, or “Mute” under “Line In”. (If the icons aren’t there, click "Start" and select "Settings," "Control Panel," then "Multimedia" or "Sounds and Multimedia." Make sure that the "Show volume control on the task bar" option is checked, and click OK at the bottom of the window).
2. Outgoing sound: Double-click on the Speaker icon. Make sure that none of the options are muted and that all levels are turned above 50%. Then, click "Options," "Properties," then "Recording" and click the OK at the bottom of the window. Make sure that "Microphone Balance" is the only option selected (check box underneath) and that the level is turned up above 50%, or halfway.
3. Close the window and try a call on Yahoo! Messenger. Better?