Sometime, I close lid, and open again, The sound not available.
When I restart my mac, The sound back.
Is this a bug?
Yes, the same bug I have from time to time, too. I could fix it without a reboot by reloading the kernel extension for audio with these commands:
sudo kextunload /System/Library/Extensions/AppleHDA.kext sudo kextload /System/Library/Extensions/AppleHDA.kext