What are you doing when this happens? Have you got Garageband installed because you can reset the core audio by simply opening then closing Garageband. Worth a try but I doubt it'll help. 🙂
That's been noticed by a lot of people, and it seems to be more of a quirt with Activity Monitor than anything. Are you having actual audio problems or just seeing that "not responding" note?
I get the same. When running Cubase or Logic I just loose all sound. I have to close them, kill coreaudio, and then reload the apps. Does it in other apps such as iTunes also.