Screen flash disabled but happens anyway

My Mac has started flashing the screen instead of beeping. It's exactly the same effect as you'd get if the Accessibility pref pane's "flash the screen" option was enabled, except that it's not enabled. The checkbox in System Preferences is not checked.
I also verified that the preferences property list file affecting this setting shows it as being off. In ~/Library/Preferences/com.apple.universalaccess.plist I see:
<key>flashScreen</key>
<false/>
In case it makes any difference, the system sound is otherwise normal and not muted. I'm listening to music from iTunes, but getting screen flashes instead of beeps.
This happened a couple of weeks ago, and rebooting the Mac got things back to normal. Why does it keep happening? Is there something I can do to stop it from repeating again? The screen flash is going to give me a seizure.

Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work.
Isolating an issue by using another user account
If the problem is still there, try booting into the Safe Mode. Shut down the computer and then power it back up. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the grey Apple icon and a progress bar appear. The boot up is significantly slower than normal.
Safe Mode
Safe Mode - About
General information.
Isolating issues in Mac OS X

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HI i was just wondering if anyone could help me!!
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if anyone could help that would be fab!!
pretty pretty please!!

Hello ,
Thanks for using Apple Support Communities.
Please follow the steps below to troubleshoot having no display on the external display.
No video or no signal, image distortion, "snow," or flickering
Check connections
When using an external display be sure to check the following:
If you're using an Apple notebook, confirm the AC power cable or adapter is securely connected to the computer and the cable providing power to the display is also secure. It is always good to have your notebook connected to AC power when an external display is in use.
Confirm display adapters are fully seated in their respective connections and that they are supported models and for the computer and display. Refer to these articles to assist you with adapter compatibility and further configuration information:
Monitor and Display Adapter Table
About Mini DisplayPort to HDMI adapters
Apple Mini DisplayPort adapters: Frequently asked questions (FAQ)
Thunderbolt ports and displays: Frequently asked questions (FAQ)
Remove all display cable extenders, KVM switches, or other like devices and retest to determine if the issue is resolved.
Try unplugging the video adapter or cable and then plug it back in.
If more than one video adapter is in use (or "daisy-chained"), troubleshoot by using only one adapter.
Example: A mini DisplayPort to DVI adapter connected to a DVI to HDMI adapter is an unsupported configuration because there is a series of adapters in use.
If available, try using a different display and or adapter (or use a different connector by using DVI instead of VGA, for instance).
Reset the system
You can reset the Mac's parameter RAM and SMC.
Reset the resolution
Start by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:
Start up in Safe Mode.
From the Apple () menu, choose System Preferences.
Choose Displays from the View menu to open the preferences pane.
Select any resolution and refresh rate that your display supports.
Restart your computer.
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N95-1
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Screen flash disabled but happens anyway

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