The audio behavior IS the spin behavior when it controls rotation.
For each shape you want to control, select the shape.
In the properties pane, right-click on (I'm assuming) Rotation > Z and select
Add Parameter Behavior > Audio
If you don't have any audio imported - import something.
In the Behaviors pane, set the source audio (select To and either the imported clip name or Master.)
Respond to: Amplitude
Graph Range: (start with) Bass
go down to Smoothness -> slide up to 10
Peaks: Sharp (or Smooth... you'll need to play with everything to get a feel for what you want.)
Apply Mode: Add; Subtract; or Add and Subtract. (Alternate Add and Subtract between different objects.)
Scale: 60 (to start)
[*what about Multiply? doesn't work with rotation - scale, position and other parameters, yes; rotation, no.]
You can change the transform property from the Apply To dropdown at the bottom. (Inspecting that dropdown will show you ALL the possible parameters Audio can be used to control ... for that object.)
[you can add multiple audio parameter behaviors to a single object and use them to control several different parameters simultaneously. You can even create an Audio behavior to control another audio behavior (for Floor, Ceiling and Scale values) ]
There are several "things" in Motion for which Parameter Behaviors (and keyframes) are disabled -- if it can't be done - Motion won't let you. For a minute, I thought I could get around it because you can add the Spin value to a Rig slider and keyframe that, and although the keyframes and the slider worked just fine - the keyframing had no effect on the settings I had the rig controlling the Spin value (if that makes any sense?) [The Rig, unkeyframed, does control the Spin value like it is supposed to... as long as there are no keyframes set for it.]