Hi!
I came across this forum because I have exactly the same problem.
I have changed the power options, as described in your post, but this does not seem to work for me.
I have sound for about 10-20 minutes after I start up my PC, then it suddenly dies and doesn't seem to get back until I restart again.
One thing I have noticed is that if I play a youtube video while the sound dies youtube also pauses the video.
I never had this problem with Windows 7, only with Windows 8.
If anyone has a any other suggestions on how to deal with this problem it'd be greatly appreciated!