That's funny. I just had the issue and this thread is the first I found. I was so bothered that the other guy didn't share his solution that I made an account just to tell him off. I was fortunate to realize the solution myself after trying to restart my PC and seeing the icon persist.
I know the answer and screw the guy who started this thread for not sharing the embarrassing solution. We have Samsung monitors with the volume on the monitor itself muted. That's all it was. I somehow hit the mute button on the remote control for my screen. But more importantly, screw user...