Controls what happens when readline wants to ring the terminal
bell. If set to none, readline never rings the bell. If set to
visible, readline uses a visible bell if one is available. If
set to audible, readline attempts to ring the terminal's bell.
I don't think I've seen a terminal with an actual bell you could ring since the Teletype days; but the original UNIX terminology lives on!
Now I have this strange urge to design a device which will ring a bell whenever the PC's speaker beeps. Do you think there would be a market for such a device?