Dammit. Rome: Total War has broken, probably by a MS Update since that's the only thing that's changed recently. (The update also seems to have done weird things, like putting me at a logon screen to click my username when the PC has been idle for a while). Tried all the traditional remedies of rebooting, uninstall/reinstall of the game, reading forums followed by ritual purification, swearing and kicking bits of hardware. Nothing's worked, how am I going to make hours at a time disappear now? I might have to do something more useful with my life than replay a game I've been playing constantly for the past year.
Update w00t! fixed it. It was the audio driver update. /me ♥ XP's driver rollback option.
Looks like I called it correctly on the "Barbarian Invasion" expansion though, not that it was that hard: Shogun:TW -> Mongol Invasion Medieval:TW -> Viking Invasion Rome:TW -> er... I wonder.