Why isn't my system producing an audible system beep in iTerm2 or Terminal?
Turns out that I hadn't selected an alert sound yet in System Preferences. Once I chose an alert sound, all three of the above methods produced audible system beeps, in additiona

To enable audio alerts, I chose an alert sound from the Sound tab of the System Preferences: 



I chose "Sosumi" out of nostalgia for my IIci days. 

Alternatively, this can be set as described in this answer, though iTerm2 needs to be restarted for it to take effect in Sierra or El Capitan: 

defaults write com.apple.systemsound com.apple.sound.beep.sound /System/Library/Sounds/Sosumi.aiff