I've had this problem since updating to Windows 8.1
After not playing sounds on the laptop for a few minutes (sometimes less, sometimes more), the volume gets set to about 45-50 until I attempt to change the volume using Fn+F2/F3, at which point is gets set back to whatever volume I was using earlier.
This didn't bother me at first, but I'm having to continually do it practically every time I pause and play a video or song.
Any solutions?