The Settings > Audio section has nothing to do with the volume on your laptop. There is no way to adjust your laptop's volume from inside Mobile PC (or any other GPS program, as far as I know).
You need to run whatever you use for volume control on the laptop. For instance, if there is a speaker icon in the System Tray you can click on it and adjust the volume that way.
(Windows XP) If there is no speaker icon in the System Tray you can go to the Control Panel, double-click "Sounds and Audio Devices" and adjust the volume. At the same time you can select "Place Voluem icon in the taskbar" so you won't always have to go to the Control Panel to adjust your volume.
(Vista) Control Panel Home > Hardware & Sound > Sound > Adjust system volume
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