MiscBell is an option that (as far as I can tell) is only documented here: https://docs.xfce.org/apps/terminal/advanced I've had issues getting the terminal bell to work in my graphical environment, so when saw no setting to enable/disable the bell I assumed it as always on and that something else was wrong. It turns out that MiscBell is a "hidden option" that's only settable in the configuration file. To reduce confusion and make the bell feature more discoverable, I propose adding a checkbox to the preferences dialog under "General" or "Advanced" to "Enable bell".
Igor referenced this bugreport in commit 02ba9b9d217e530c140ee710d45d7e2a17fa7785 Add Audible and Visual bells configuration to Preferences https://git.xfce.org/apps/xfce4-terminal/commit?id=02ba9b9d217e530c140ee710d45d7e2a17fa7785
Implemented, thanks!