Windows vista does not play any audible sound when your laptop reaches a critical battery level rather it displays a small pop message which most of the people miss many times.
Laptop Battery Alarm is a FREE software which lets you play an audible alert when your laptop battery level drops to specific power level which you can specify. You can also set it to repeat the alarm after every 5 minutes.
As you can see in the image above this small utility sits silently in windows system tray and play notification sound when your laptop battery reaches a critical level.
Laptop Battery Alarm is small easy to use tool, does not require any installation but runs simply by clicking the executable of the program. You can also select a different wav file to play as a notification sound for low battery.