Sound falls out

It's really strange:
Sometimes when I activate my pc, I dont get any sound:
The strangest thing is, WMC says it is playing music, but it isn't going
forward.
I can't get it working, before I restart my computer.
there isn't even game music.
Sometimes WMC plays half a song and drops out like this.
I have checked everything out what I could think of: drivers, speakers error
in the institutions.

My soundcard is a Creative Soundblaster X-Fi Xtreme Gamer Fatal1ty
Professional.
And the speaker set a Creative Speakerset Inspire T7900 7.1
So that shouldn't be the problem.