MOV to .264 audio falls out of sync?

I exported a Master File from FCPX and when I play it back everything is perfect. I drop the MOV into Compressor and select Blu-Ray.264 and .ac3. After It processes, when I try to put together my BluRay disc I notice that over time the audio and video have fallen out of sync. How can I correct this ?

Be sure what you are working with inside FCPX is 48kHz sample rate audio. Can we verify that?

The audio is a mixture of 44 and 48.

There's your problem. All audio in video editing must be 48kHz. If you are using 44.1kHz sample rate audio, it will drift out of sync with the 48kHz sample rate clips.

Ok, I made sure all the audio is 48kHz but I'm still having this problem. If I export to a Master File, the audio is in sync. But once I convert to .264 the audio starts to fall out of sync as the video plays on. Any suggestions ?