Having got fed up with not spotting the low battery warning flag on my laptop and losing work etc etc , I searched for a workaround.
Apparently the lack of an audible warning in Vista is an Operating System problem and someone has come up with this piece of software:
http://batteryalarm.codeplex.com/
You can configure it to:
1. warn you with any .wav sound file
2. choose the point at which it warns you - eg 5% charge, 10% charge etc etc
3. choose how frequently to repeat the warning - as per the 'snooze' facility on an alarm clock
It doesn't start-up automatically, so I just copied a shortcut in to the start menu folder - works fine for me