I’m doing the same thing. If I get a FCP project file I make sure to create an offline with sound, and ‘extract audio’ in Resolve from that offline. I do the same thing if I get only a big QT file too.
I believe I read somewhere that Resolve won’t play more than stereo. But if it could to 5.1 it would be super awesome.
I’ve recorded a few grading sessions and play-outs on my other Mac Pro with Media Express. There’s clearly 8 channels through SDI.