What's wrong with the volume on my computer?

All the jacks are plugged, the speakers are working, the volume icon is displayed properly on the screen, the level indicator is going up and down like its working, and when I check the working order of the device, by right clicking on the icon, it reads it working normally-The thing is, one day it works fine, then the next day when I turn the computer on, the volume won't work, and then it works again---HELP!