For my StepMania modded PC, I used a sound sensitive LED rod. It works amazingly well.
Here's a pic of the front of the mod - the LED rod is in the window where the radio tuner would normally be (you can see the rod behind the translucent mylar I put in there for diffusion).
The only downside of using a rod like that is that the reach of the USB cable is pretty short. Plus, the controller box that contains the microphone needs to be close to the speaker and turned to a low sensitivity in order to make sure it is lighting up only when the bass beats hit.
In my mod, I put the controller box directly on the back of the main bass speaker (it's the small blue box in the middle).
By the way, I'm looking to sell that machine if anybody's interested.
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CF cbox mods and more at the 'Pad'