Sounds works sometimes, then not, then again, ...

I've read interesting posts about realtek audio on Vista. I have a similar
problem as most of these people, except that I don't connect my WMCenter to
speakers, but to an amplifier.
THe connection cable is an optical one. So realtek audio displays an icon
for "realtek digital output" and one for "speakers". Through trial and error
in the past I found out that the first is for the digital connection (optical
cable), the latter for the analog connection (red/white cinch).
I've had problems in the past, which I was able to resolve by playing with
enabling and disabling devices or rebooting the PC, reconfiguring the sound
in the WMCenter application, ... but it was never clear what exactly resolved
my problem.
This time the problem occurred again: I listened to the radio on my WMC this
morning, switched off the PC (standby kind of mode on the ACER Media center
PC). When I wanted to listen to the radio again tonight no sound to be heard.
(not only in te WMCenter application, but no system sounds, nothing). I've
done the same checks as always. Everything should be working and the little
green "volume" bars next to my realtek output device icon in the "sound" box
move nicely up and down when playing test sounds, but no sound to be heard....
Any idea how I can resolve this problem once and for all??