OK, there are a few things you can try:

1. From Yahoo! Messenger’s main menu, select Preferences. In the Voice & Calls section, check the Speaker and Microphone settings. You can test your selected speaker by clicking the Play button next to it (it’s a little arrow). And, you can test your selected microphone by speaking into it: if it’s working you’ll see an animated level indicator.
2. Check your system volume next. Single-click the Speaker icon in your system tray (in the bottom-right corner of your screen) and drag the volume bar to the top. If the little speaker icon below that volume slider has a red circle with a slash through it, your sound is muted. Click that speaker once to turn off muting.
3. Did that work? If not, select Mixer (below the volume bar). Make sure that none of the options is muted, and that all levels are turned above 50%.