Solution 1:
Run in a terminal:
gsettings get com.canonical.indicator.sound visible
If the outcome is is "false", change it with the command:
gsettings set com.canonical.indicator.sound visible true
Run in a terminal:
gsettings get com.canonical.indicator.sound visible
If the outcome is is "false", change it with the command:
gsettings set com.canonical.indicator.sound visible true