Look at the driver properties. I suspect a generic Microsoft driver has been installed and that driver doesn't work correctly. (Right click on the audio device from Device Manager and select Properties. Then check the Drivers tab. Look for the line with Driver Provider.)
I can't find a MX6400 listed on Gateway's driver site,
http://www.gateway.com/gw/en/US/content/drivers-downloads . There are the MX6410 and MX6420 listed. Both of these have a Conexant audio driver for XP and Vista. The Vista driver may work under Windows 7. (Make sure to correctly select the 32-bit or 64-bit depending on your OS version.) Download the driver and try to manually install it. If that doesn't work, try a Conexant audio driver from another laptop brand that is for the correct version of Windows 7. (Just Google Conexant audio driver Windows 7.)
I hope this helps. If you have a problem, please let me know what your Device manager claims the manufacturer is on the HD audio device and the driver manufacturer listed in the Properties. Also add a comment if you see any error messages when trying to install a new audio driver.
Cindy Wells
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