If you have speakers on your monitor, before or after the change, then you may need to change their settings in your sound card. Or if they're external speakers, that may have been changed as well. Of course, it's also important to make sure you didn't accidentally unplug the speaker cabling to the back of the computer.

The easiest way to check for speakers and sound/volume settings is by RIGHT clicking on the speaker icon in the lower right corner of the computer and first choose "Playback Devices", then make sure the correct type of speakers are chosen, the correct playback device is listed, and all are enabled. Close the window. Next, RIGHT click on the speaker icon again, choose "Open volume mixer" and make sure all volume levels are correct and aren't muted.

Hope this helps.