HP Pavilion Desktop with Vista. Sound suddenly quit working. Is the audio card replaceable?

Sound was okay. Heard a "click" from the processor. Later discovered no sound. System Restore to point before updates did no good. Checked in Device Manager for audio card driver, okay and updated. Checked in "Sound", it says speakers are working. Checked volume and "mute", no problems there either. Speakers are external and do make a noise when wire is unplugged, so I don't believe they are the problem. Anyone familiar with this computer, HP Pavilion Desktop CTO model d4990y? Is the audio card replaceable or is it integrated with something? Can find little information on this computer, even on the HP support site or other searches. Would appreciate any information or knowledge on this computer problem. Thought I'd ask before I attempt to remove something that shouldn't be! No problems for years and then suddenly, no sound. Any help appreciated! Thanks very much!