What is coreaudiod?

For the past day or so I've noticed that I no longer hear the volume beeping sound when I adjust the volume on my mac, but I still get sound. A few minutes ago it started to get really hot and the fan was running hard even though practically nothing was running. I just checked it out and there was something hogging my CPU called coreaudiod. What is this, why is it here, and how can I fix this?