Suddenly no sound, no microphone & video issues

I suddenly have no sound, the microphone doesn't work and videos won't play online (youtube, etc.). I found out while trying to Skype that there was no sound and the microphone doesn't work anymore (Viber also doesn't work and neither does the call function via Facebook). I have used Skype before on this computer and it worked fine then. All the devices (drivers) are working properly (no yellow exclamation marks of doom) and I haven't changed any settings that I know of. What on earth is going on? I can't even listen to any music! (and yes, I did check that the volume is up).

Thanks for the suggestion but nothing's changed, even after a System Restore. What else can I do?

Thank you so much, that worked!! The model number is actually R505C (will change that in my details).

Your suggestion, fireberd, did work - except that suddenly everything went silent again yesterday (I only noticed because I wanted to watch a video): I did the same thing you suggested above and it worked again. My question is: why does this keep happening? I've only been using my computer for routine stuff (email, word docs, Firefox, Solitaire to stave of the madness, etc.)

I'm still having the same problem - if I uninstall the sound driver, restart computer, let it reinstall the driver, the sound works. But as soon as I turn the computer off again, I have no sound until I repeat the above steps. I'm not using any programs that deal specifically with sound (last time I had the following open: Thunderbird, Firefox, Open Office). Could it be VLC that turns it off? It's how I've been checking if the sound works - by playing a video through VLC.