When you adjust the volume with media keys a pop-up (OSD) volume control appears in the top left of your screen showing the new volume level. You can also click on it once it has appeared to quickly set the volume to a new level.
I have noticed that sometimes if you adjust your sound device settings this volume control can get stuck displaying a previous value - for the sake of example, let's say 50%. When you adjust the volume via the keyboard the system volume does change, but the OSD displays the new value briefly before flickering back to the previous value. If you were adjust the volume up from zero it would display 0%-50%---2%-50%---4%-50%---6%-50% etc. If you click on it in this stuck state it changes the stuck value but doesn't actually change the system volume level.
I can replicate the issue by unplugging my primary (and in-use) audio device then plugging it back in. I have four audio devices in my system:
- SB X-Fi Go! (USB, primary device, the only one that gets daily use - using Creative's drivers rather than stock USB audio ones).
- Rear-panel motherboard audio ("High Definition Audio Device", S/PDIF).
- Front-panel motherboard audio ( "High Definition Audio Device", headphones).
- HDMI (Radeon HD6770).
Rebooting the PC corrects the issue. The volume control in the charms bar is not affected.