I have an interesting project where the 5.1 files and being supplied to me before the edit. I know it's not the best way to work, but that's what I have. Ive done this before on Avid and was very careful not to mess up the sync.
Is there anything I need to look out for besides locking the files?
Do I have to warn the client that there could be issues?
It's a 2 hr concert. I have to add a few wide shots and cut a few things out.
Main issue importing any audio files into FCP is to make absolutely sure that your Easy Setup default project has identical timecode frame rate as the project you will be editing in. Otherwise sync drift can happen.
So set the right Easy Setup. Start new project and make sure you understand the audio routing. If you want to monitor as 5.1 then you need to have 6 channel mono output and patch the tracks to the correct output.
If you want to make a stereo foldown then make sure you use stereo output and the tracks are correctly panned. Drop the left and right surround channels by at least 3 db each so the foldown sounds more normal. You might find you have to drop gain on all tracks as the foldown might overload so just have the surrounds down an extra 3db.