Yes, same sample rate.
I think it will be interesting to repeat the same work with Premiere instead FCP. I will test.
Thanks.
I don't know enough about how audio programs handle project clock rates (or frame rates) to talk about them. But all video programs I know of are locked at the project or sequience level to a frame rate. (If they were not, undercranking and overcranking would have no meaning or predictability., and audio would never stay in sync.)
When video is imported, if the project framerate (sometimes called base framerate) of the video files don't match the project rate they are being imported into, the video will have issues. It will skip frames or have repeated frames from time to time, depending on the naturer of the mismatch.
When a WAV file is imported, it has no frame rate, but since video programs operate on frame-based time system, the program "assigns" it a framerate (sort of) specifically the project frame rate. What that really means is that the editing program makes the audio file's clock coherent with and addressable in frame terms, so that video-frame-based editing can work and everything will stay in sync.
Separate issue--the audio sample rates need to mach the project rates on import as well, or conversion will be required, and will usually happen "auto magically", with varying results. :-)
Syncing, then, is a simple matter of finding a frame on the two sources that are known to be happening at the same time and telling the program those two files are in sync at that point. (That's what slates with clappers are for, though there are other ways to do it.) With any video camera I've ever used (and that's a long list), and any audio recorder worth using, there will be no appreciable drift or time mismatch once sync is established, provided that the project rates are all correct on import. (Here I have to say that my use of FCPX is pretty limited. I do all audio syncing in Resolve or RedCineX at this point, but I have done lots of syncing in Premiere and Avid, and I expect that FCPX handles clock references on import in a similar way. But what's likely is that FCPX hides the functions, so that it's not obvious what it's doing, and therefore it'll be much harder to troubleshoot if you're doing something wrong.)
Where you run into problems with audio sync is when you use a camera set to 24fps, for example, and put that material into a 23.976fps project. You can tell the editing program to see it as 23.976, but the video is no longer running at it's original speed, so all audio will be out of sync unless you change its playback speed as well. (That used to be the norm for film-based TV in the US, because film cameras were clocked at 24fps, and were transferred to video at 29.97fps interlaced, which, even with 3:2 pulldown didn't quite match, so all the audio had to be cross-reference resolved to match the slightly slower playback speed.) That's not ben necessary for a long time--even with film--but if you want to avoid all that, you have to decide what your deliverable is at the beginning, and keep that as the clock rate for everything throughout the process.