Hello - the Audio parameter behavior will do exactly what you want - it's almost exactly like soundkeys. Here's a little movie about it:
http://cdn3.libsyn.com/macbreak/macbreak-087-450p-h264.mov?nvb=201005270030...
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Mark Spencer
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press
http://www.applemotion.net