First, check the LCD monitor's manual and manufacturer's website for errors such as this and their resolution. (you may also want to do the same for your sound card. Won't hurt to make sure that all your drivers are up-to-date, either.)
If that doesn't solve anything, try this:
In the control panel, go to the "Sound and Audio Devices" panel. On the "Audio" tab, ensure that your sound card is selected for the default playback device. If it is, or there is only one device to choose from, click the "Advanced". Ensure that your speaker type is selected under "Speaker Setup".
You should also check to see if your LCD is listed under "Sound, Video, and Game Devices" in the Device Manager and if it is, try disabling it (make sure you don't disable your video card or monitor, though.)
Let us know if that works or not.
Edited by Amazing Andrew, 12 December 2006 - 07:37 PM.