How to get unique low battery warning sound?

All the warning/alert sounds in OS X seem to be the same sound, so it is very confusing if you are running a full screen application, or simply can't see the alert window or popup for some reason. Often, I maybe watch a video while connected to the internet and hear the alert sound and I have no idea what it wants to alert me about.

So there seemingly are no sound themes like in Windows XP, but is there at least a way to get a unique sound when the battery is about to run out?

Criterias for answering this question is that the solution must not require Dashboard, Konfabulator or a non-freeware application.

Thanks!