This is from a thread I just replied, only copy and paste hope will answer your question - if not you can read the entire thread see if it helps. links at the bottom.
"The reason why the option is grey out is because when capturing the footage, the sound was captured as dual mono instead of stereo pair. you can change the setting in the capture window under clip settings. the way to varify if your clip was captured as dual mono or stereo, double click on the clip in the browser, when it pops up in the viewer see if the audio tabs says stereo or mono. if you must do a send to soundtrack audio file project, here's the way to do it: unlink the footage, then unstereo pair the two mono, and select only one audio track by itself, then right click and go to "send to" you'll find that the option is now available. You could also do send to soundtrack multitrack, and then double click on the sound clip once soundtrack pro load up, you'll get to the same window. Hope this helps.
explanation - in apple support you will find an article stating that you cannot send to soundtrack for audio file project when the clip are linked with medias from different places, that is the reason to unlink. For some reason, when capturing the sound as dual mono, even thou it's coming from the same tape captured at the same time, final cut thinks dual mono as two different "entities" "
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