First of all, the Audio parameter behavior is not that great. It works best when you can separate every instrument into its own track (individual file!) Then there's the issues of harmonics (which are invariably produced) and "spectral crosstalk" (reverb depending on surroundings), etc. And the list goes on. It would be SO much better to have MIDI file control (notes on/off, program and control changes, at the very least). But we don't have that... So in the meantime:
If you create a project to be published for FCPX, with audio, Motion will put the audio file in the Media folder of the effect you published.
What happens when you create an animation based on Audio parameter behavior is that Motion analyzes the audio and creates *Keyframes* for the object parameter under control. You can actually delete the audio once Motion has done this, it will not affect the animation whatsoever.
So, what this means is that you only get to create one published effect for one audio track. Not a big deal. There's a lot involved in creating "other" projects with audio. Mostly, very few audio tracks have the same length and so you have to create a new project with a different design length every time. Neither Motion or FCPX will "scale" the project for new audio. Also, since Motion writes keyframes, the old keyframes have to be overwritten (which happens when the new audio track is "analyzed" — and sometimes getting Motion to do that correctly can be just a little bit challenging ).
The problem for FCPX is "how does it adjust to the user changing the length of a template?". It will scale keyframes, but does it speed up/slow down the music?? Usually not a desirable effect. FCPX does not initiate a reanalysis of the audio. Motion doesn't do that either! You've got to trick it into doing that.
Creating a published generator (or title) with audio is not hopeless, or all that terrible. Create the animation. Load up the audio from the Media folder and sync it (you can set a sync point in Motion if needed - like a flash frame or whatever - just provide the parameter to hide it in FCPX for rendering). I've done a bunch of "visualizers" in Motion. It can be a lot of fun! Here's a couple of them:
(piano keyboard - every key has its own audio behavior tuned to their frequencies)
https://www.youtube.com/watch?v=WNtyb_muGbc
(analog vu meters:)
https://www.youtube.com/watch?v=YT8gIzMcmq4