This app can make your n900's led being sound reactive
You can choose the colour of the notification led and which keyboard leds to blink like the sound! You can choose from 3 sources(Monitor/Bluetooth/Microphone)!

Here is a small video of it in action!

Credits goes to nicolai and you can find the program on extras-devel
(http://maemo.org/packages/view/ledpulse/)