I had this same instance happen recently.
(the volume icon in taskbar dissapeared, no win startup sounds, etc.)
I formatted C: drive, re-installed windows, dnld updates, same prob.
The second time i re-installed windows, i checked every syep for sound before continuing to next. ie: (between every update)
As far as I could limit it to was a corrupt win .netframework install
Do you have this installed? - .netframework?
I am not positive, but this is as close as I could confirm it to.
Even when I had no sound, the evice manager would show devices working properly. ie: no yellow exclamation marks, yet it wouldn't work.