[SDPhantom]

These functions exist, but don't have any slash command strings registered for them.

SlashCmdList["UI_ERRORS_OFF"] = function(msg)
    UIErrorsFrame:UnregisterEvent("UI_ERROR_MESSAGE");
    SetCVar("Sound_EnableSFX", "0");
end
 
SlashCmdList["UI_ERRORS_ON"] = function(msg)
    UIErrorsFrame:RegisterEvent("UI_ERROR_MESSAGE");
    SetCVar("Sound_EnableSFX", "1");
end

FrameXML\ChatFrame.lua:2379

You can call them using these commands.

/run SlashCmdList.UI_ERRORS_OFF()
/run SlashCmdList.UI_ERRORS_ON()

Note: msg is the string of arguments after the slash commands, usually an empty string for commands that don't use them. These functions don't even reference the variable so it's safe to omit it entirely.