You can change the sound of your outgoing voice simply by talking louder or softer. However, if you're looking for ways to change the sound volume without adjusting your voice, you can do so through settings on your PC. Just follow these steps below:

1. Left-click the Speaker icon in your system tray (on the bottom-right corner of your screen) one time and raise the volume bar to the top. Make sure "Mute" is not checked. (If the icon is not 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. If that's not sufficient, double-click on the Speaker icon. Make sure that none of the options are muted and that all levels are turned above 50%.
3. 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 and the level is turned up above 50%.
4. Close the window and try to use Yahoo! Messenger with Voice.