Here is a surprise success story with FCPX that I wasn't expecting. I was given two HD ProRes versions of a European feature movie slated for limited release here in the US. Since there were no 35mm prints with subtitles the distributer was going with DCP files with embedded subtitles. Unfortunately some independent cinemas do not have DCP servers so I was asked to make BluRays with 5.1 sound for them. Due to some issue the distributer only had a subtitled ProRes 2 ch. stereo version. My job was to get the 5.1 track onto the subtitled version and make exhibition BluRays.

Knowing that FCPX defaults to 5.1 sound I decided to take a chance. The ProRes file with the 5.1 sound dropped in perfectly with all the channels assigned properly. Even if they weren't, fixing that would have been accomplished in a few minutes, It was easy to overlay the subtitled video track, check to make sure that all was in sync and export a fixed ProRes file for BluRay encoding. Happy me, happy client.

Nice, although I'm not sure how I would have done this in FCP7 for comparison.

Are you saying that you converted the stereo track to 5.1 surround? Or was the 5.1 tracks supplied to you?

The unsubtitled file was in 5.1 surround but for whatever reason the subtitled file was in 2.0 stereo. The distributor had promised 5.1 to the theaters and there was no way to get a new subtitled ProRes file from Europe. I simply replaced the unsubtitled video track with the subtitled one.