FCP7 on Yosemite: A/V sync trouble

Other than to point out that the new OS doesn't support FCP 7...nor did the previous one (FCP 7 was discontinued 4 years ago)...I can't really say. FCP 7 was not taken into account at all when the new OS was written. IN fact, Apple is moving away from Quicktime...the core of FCP 7...and moving towards AV Foundation...the core of FCX. The professional level, fully featured version of QT, Quicktime 7, isn't included with Mavericks nor Yosemite...only the stripped down, feature lacking QTX.

FCP 7...lacking the core of it's being, QT 7...might not function properly.

So if you are going to continue to use FCP 7, I suggest keeping that older computer running the older OS. If you need to get new computers with newer OS versions, then you need to keep your eye on the crop of current editing apps that work with them.

What is the format of the source files? codec, frame rate, pixel dimensions.

The audio is wrong. Well, it's OK, just not optimal for sync. 44.1 is CD quality...production prefers 48kHz. Not sure what that's working with the old FCP and OS. Convert the audio to 48kHz and try then.

and 32 bit and .wav files files might also be problematic. 48k 16bit aiff are ideal.

You cannot downgrade the OS on newer computers. The older OS versions lack the drivers needed for the latest hardware in those computers.

The thing to try first is to convert the audio to something that works best in FCP. 16 bit Stereo 48k is best...32 bit will work, but it must be 48k, cannot be 44.1

And make sure your easy set up frame rate matches your sequence frame rate. Every once in a while, you can have synch problem when you import audio only files if you have an easy setup selected with a different frame rate.