Okay... I've dealt with this since ST came out.
Here you go....
If you can't not get the option to send to to STP as an audio file
1st try to mono your audio in FCP. Uncheck the stereo pair from the
Modify menu. This separates left and right audio.
2nd You have to option click the Right or Left audio track, then
click send to STP.
I believe this all has to do also with how you captured your clip.
In my current project I captured in mono. Well with the problems
FCP has sending to STP I think I will always capture in stereo now.
You can always separate later...
Hope this helps.
John