this is maybe a Sonar a limitation right now.
I'm sorry, everything in SONAR is 100% perfect.
(annoying isn't it?)
Seriously - have a look at Ferret 4 X 4 - at the bottom
http://www.eareckon.com/en/products/freebies.html I've never tried this in SONAR - but it might do what you want - as you could split an audio track and maybe use one of these outs - to input in to FM8. It's worth a try. There's another one called Polysher - which does MIDI stuff. Maybe you could wrangle something out of it?
Also - try ticking or unticking the "use as a synth" box in the FM8 plugin properties?
Don't forget to rescan and reset all your plugins first though...