System Sounds Refuse to be Disabled

The specific irritation I am dealing with at the moment is the fact that I have not opened Outlook (2010) on my laptop in ... well, a long time. So, now I have a tonne of new email notifications pouring in, and they will not shut up.

First, I tried setting the Windows New Mail Notification to 'no sound': no effect. I restarted Outlook with no effect; I restarted my PC with no effect. Next, I tried disabling all the system sounds: no effect. Again, I restarted Outlook and the PC with no effect. Then, I remembered to check Outlook's localised settings and unselected 'play a sound': no effect. Once again, I restarted Outlook and the PC with no effect. (Also, this problem is not isolated to Outlook because, after each system restart, other system alert sounds have made noise).

I am at a complete loss. I could simply mute the sound, but I am trying to watch some DVDs. This is, in fact, an old problem that I could not be bothered resolving previously, but now I want it fixed.

Yup, that is exactly what I thought, too; however, my sound settings match your image, and yet system sounds persist. Any ideas as to what the problem could possibly be?