Well, uh, a couple of things may fix this. These are things that have fixed it for me over the years:
#1: Restart. The Windows solution to anything and everything. Pretty sure it's on the first page or so of the Hitchhiker's Guide to the Galaxy.
#2: If you right click on your sound and select: Open Volume Mixer, you'll discover that Windows has different volume controls for different things. Sometimes it'll change rapidly. You can adjust YouTube and Skype separately for example. This is what I would try first.
#3: It could be a third party program or your sound card. Try updating your drivers/fiddling with your sound card's program. Also try closing different programs and see if it comes back to normal from that.
#4: This is going to sound stupid but have you tried changing the volume from the headphones? Sometimes all my sound will go quiet and I need to adjust the sound from my keyboard/Logitech Headset to get it to go back to normal volume.
Well that's all I can think of for now. I'll post if I think of more. For now, good luck, and my Octavia grace you with her presence.
Should probably merge this into my first post, but I'm not sure if you'd then get the notification I'd quoted you. EDIT: And then it merged automatically! Like magic! Love this forum. xD
Just in case you haven't checked, Windows 7 keeps a separate volume control for different devices. My TV, for example, is set to 100%, while my headset is set to 15%, if I switch to one or the other, the volume will change. If you're using an external sound system, such as a TV, check to make sure that its separate volume controls are also high enough.