A popular effect that's often requested, VU meters that move in sync with the audio track. Here's a good tutorial showing how that's achieved using the audio and quantize parameter behaviors.
FX Mahoney has built this tutorial to show you how to build a pair of stereo VU meters in Motion. First interesting point is that the audio track has to be imported into Motion as you can't build this as a template and use it directly in FCPX. This was the same in previous versions of Motion, but with the tighter integration of the two apps hopefully this will become unnecessary in the future.
A great tutorial, but why stop there? Breaking the audio down into frequencies would give you a rather cool graphic equalizer effect. These aren't meant to have any metering usage, they're to add a bit of 'eye candy' to motion graphic projects.
Once again thank you to FX Mahoney.