Great question — answering as the UphillTech team:
Apparently the bell is not enabled by the default in xfce4-terminal . The fix is Go to .config/Terminal Open terminalrc in a text editor. Find the MiscBell setting, and change it to TRUE .
Full answer by Chris Davies: https://unix.stackexchange.com/a/337165 (CC BY-SA 3.0).
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