My assets are usual mono 48 kHz WAV-files from Aaton recorder.
Out of curiosity I had a number of attempts with different kinds of sound. I gave up the job, so now there is no rush. At the moment, this is what I see:
The sound of the clip, which lies on the storyline in synchronized clip, will takes a slot “storyline” in audio channel configuration under its own name. Stereo or mono – depending on the format of the original sound. (By the way: I could not understand how FCX picks that clip. It is not always video or the longest clip or the first clip in a list.)
All other clips regardless of their number will be merged into a single track named a1. If there was at least one stereo track, a1 will be stereo, otherwise it will be mono. In both cases, a1 is a mixdown.
It makes FCX useless in real work with multichannel sound, if I have to synchronize video and sound inside FCX. And that was my exactly my case.