This *may* only work if the HDMI cable is currently connected to your HDTV and the HDMI port is enabled (you have your desktop on the HDTV).
Running Windows 7, it should be just a matter of clicking the Start button. In the search window, type (without quotes), "Manage Audio Devices". This should give you a number of options. Click the option, "Manage Audio Devices".
This should bring up the "Sound" window. If the "Playback" tab is not the current tab, click it. This should list all available sound devices installed on your laptop. One of these options should say something like "Digital Audio/HDMI". Select this option and click the "Set Default" button.
If you have any troubles beyond this or the instructions don't seem to match what comes up on your screen, let me know.
-Wolf sends