I've played through Throne of Bhaal many times before but never encountered this particular bug:

After reinstalling Windows, I loaded up my save game and suddenly my main character is perma-silenced. Nothing shifts it, including dispel magic, casting Silence 15' Radius in the hope of overwriting the flag and then waiting for that to vanish, unequipping all my stuff, resting for weeks - I can't say a word. As a mage this is somewhat of a problem, heh.

Shadowkeeper has a tab called "State Flags" which contains a Silence flag, but it doesn't appear that you can edit it, and even if you could I wouldn't know what to do. Does anyone know how to give my mage his voice back?

Hey, welcome.

Have you tried "Heal" on the character? If none of your clerics can cast it, try activating the cheat keys and use CTRL+R on your character.

EDIT: You may also try casting "Remove Curse" on your silenced character.
http://www.sorcerers.net/ubb/ultimatebb.php?/topic/10/2851.html#000000

I hate to state the obvious, but have you tried Vocalize? There are also a number of items that you could put on your mage to prevent it from ever affecting him, including the Amulet of Power.

Thanks for the replies. I've tried Heal, that doesn't work either. I have the Amulet of Power equipped, which supposedly has a permannt Vocalise effect on it, but that doesn't do anything, and nor does taking it off then re-equipping it.

However, Remove Curse worked a treat. Thanks very much!