Hi there, hope i can help. You actually have several options here, so lets see if we cant get your sound back up.
1. Have you tried System Restore? You may be able to go back to a date before this problem occured to restore your sound.
2. Creative Labs makes a Sound-Blaster that is USB powered, this is a sound card that you can upgrade to by simply plugging it into your USB port.
3. Just cause your volume is UP, does not mean your volume is UP
there may be a 3rd party software that actually is in control of your volume, I know on MY laptop I have installed other software that take charge of my sound properties. you may have inadvertently done the same. Skype may be the culprit here.
Hope this helps, but try the System Restore option first, it may be the one that does it, if not, see if any new software you have installed is actually taking control of your sound properties. Cheers.