If you exportred the video to a .m4v (iTunes video) change the suffix to .mp4. YouTube for some reason really doesn't like the .m4v suffix and it mangles the audio sync all the time.
I export a lot of HD Video to YouTube and I find I get the best results from expoting the movie out as a stand alone video to the desktop, opening it up in Quicktime X or 7 and using the save for web function choosing the desktop setting.
I then change the .m4v to .mp4 and upload that to YouTube.
If I don't need a stand alone master for back up I use the iTunes option under the share menu in FCP X. The converted video iis put into your iTunes movie folder where you get get it, change the .m4v to .mp4 and upload that.
Don't forget to delete the video from iTunes unless you want to keep it there are a reference.
By doing this I end up with great looking video, no audio sync problems, and its much faster than using compressor.