"System Sounds" mute button is not consistent with slider
If you move the "System Sounds" slider to its zero position, the mute button gets toggled. But you can still untoggle the mute button, leaving that setting in a inconsistent state: slider at zero, but mute button indicates it is not muted.
This is very clear too in #459, when you open the panel for the first time and the "System Sounds" slider is at its zero position but the mute button is not toggled.