The Radeon 4000 series cards include sound capability (output to the HDMI adapter) so if you added a Radeon you might now have two sound devices competing with each other. I disabled my Radeon 4850 sound by using Control Panel, System, Hardware, Device Manager and clicked on ATI Function Driver for High Definition Audio and selected disable (it's under Sound, Video, and Game Controllers- at least in Windows XP). It's a long shot and probably won't fix your problem, but if you aren't using the audio capabilities of the Radeon graphics card you might try disabling the audio part.