Weird audio sync issue...

So, I've shot a bunch of dance performances in a studio.
And on one of the performances the audio don't sync.

What I usually do, is that I film them, with the cameras picking up the sound in the room, then when it's time to edit, they give me the music file. Since the music files generally are MP3s from CDs or the net they are 44.1kHz so and as the camera records 48kHz, I convert the MP3 to a CAF Apple Lossless 48kHz file before importing to FCPX. This has worked great for 50+ edits.

But for this one, the files don't match. The audio in the camera is longer than the MP3 audio. It's almost like the song wasn't played at the right speed at the time of the performance. It seems like the amount of difference that it would be playing 44.1/48 files without proper conversions.
I tried to drag an unconverted 44.1 file in there but since the FCPX project is set to 48kHz it seems like FCP automatically converts the file as it ends up identical to my converted 48kHz file.
Is there a way to make FCP not convert the files?

Any other suggestions?