Users of real Macs have this issue too - losing sound randomly - seems to have to do with AppleHDA ( I use vodoo and don't have the issue at all , or better : not happened yet, who knows ..)
coreaudio is going mad in Lion as it seems.
try : sudo killall coreaudiod to restart the daemon, resetting sound.
this guy blogs about it :audio issues in lion
and this guy created an app to fix it :audio fix dmg
But it doesn't seem to help me. I've now reinstalled Lion like 4 times and each time I'll get the audio working with ##### and chimera. And then the next reboot it will be gone. I tried the app that guy made and the sudo command but it says no matching processes found.
I've been running voodoo 0.2.1 with teh HDA rollback to see if I can't get it running again.