How do I disable the system beep in FreeBSD 10.1?

If you're running vt a.k.a. newcons, try:

kbdcontrol -b quiet.off

If that works, you can make it permanent in your /etc/rc.conf:

allscreens_kbdflags="-b quiet.off"

Background:

After running kbdcontrol from an Xterm and seeing it print out an escape sequence, I realized that it is just trying to send a command to the terminal emulation in the console driver, and it might need to be sending something different depending on the console driver; then I looked for and found the answer specific to newcons: http://lists.freebsd.org/pipermail/freebsd-current/2014-April/049463.html

Use sysctl -a | grep bell to see which knobs you've got. vt is probably what is being used these days.

sudo sysctl kern.vt.enable_bell=0 works in my case.