Unfortunately, Motion doesn't currently have expressions or any other form of parameter linking. That is needed to do what you want. BUT... specialcase wrote an utility which converts After Effects position keyframe data to Motion position property. You could use After Effects' Convert Audio to keyframes assitant, copy the resulting keyframes to the position property, and then use specialcase's tool (you will have to copy and paste the position keyframes to the property you want).
You can write a plugin using FXFactory & Quartz composer that will take audio amplitude or even frequency and modulate a parameter.
Take a look at the cube I have on my homepage http://www.peterwiggins.com/
Does exactly that, you can open the thing up in quicktime player etc.
here is the bad news - although it will play in motion, when you render the timeline doesn't move so there is no sound, so it doesn't move.