As long as your drivers are up to date what you will need to do is switch the default audio device to your HDMI audio out in windows (Control panel-> Sound -> Playback Devices [Win7/Vista]).
But this also assumes your hardware supports audio out via HDMI. As we don't know what your laptop is or your OS..
Thanks, haveAcer aspire 7735Z-4613 Windows 7 Ultimate. Cannot see anywhere in where HDMI is mentioned in control panel