Between Jaunty and Karmic, a number of changes were made to keep the PC speaker from beeping. As part of this, system bell events are now captured by metacity, which uses libcanberra to play a sound. For users without speakers, this fails to be useful. The current setup makes restoring the old behavior extremely difficult.
The absolute show stopper is that metacity traps audible system bell events. This behavior is, as best we can tell, not configurable. The attached patch keeps metacity from capturing system bell events. It also removes the sound playback capability. As Lucid will be using pulse audio's module-x11-bell to play sounds for system bell events, it is not necessary for this capability to be in metacity. Additionally, this removes the discrepancy between metacity's and compiz's handling of system bell events.
There are several other difficulties in enabling the system bell:
1) Even if it is taken out of the blacklist, the pcspkr module may not load properly. Another modprobe may be necessary to get it loaded. See bug #398161.
2) Something in Gnome's startup does the equivalent of `xset b off`, so `xset b on` must be run on every login. (Note that bug #280767 keeps you from setting the bell volume with xset.)
3) A number of settings in gnome-terminal and gconf may keep the shell from sounding system bell events.
Comments #28, #34, and #50 give more detailed summaries of the various problems.