Well, the easiest "fix" I can think of is to create a launcher to run the command I gave, so unmuting the sound only takes a click... But like I said before, it's not really a solution - perhaps the problem will be fixed in an update, or perhaps you'll work out what it's upset about
Maybe searching through /var/log to see if there are any relevant messages.
Anyway, to add a launcher:
Right-click on a panel (if you don't know, this is the GNOME equivalent of the Windows 'taskbar'. It's at the bottom/top/side of the screen
Choose "Add to Panel"
Choose "Custom Application Launcher"
Click on the icon of a spring and find a nice volume-related icon - "/usr/share/icons" has lots, I picked "/usr/share/icons/hicolor/scalable/multimedia-volume-control.svg", but this might not be on your system.
Enter Name: "Unmute volume"
Enter Command: "amixer sset Master unmute"
Optionally, enter a comment.
If you click on this launcher it should unmute your volume