I've noticed this problem recently as well, though I haven't isolated it to the Mac... it seems to be an issue with my more recent .m4v content. I believe I have an audio setting incorrect during my rip process. Older video files don't seem to have the video lag.
Nonetheless, for each video, you can change the audio response to either sooner or later. Kind of a hassle, but it's a fix. My only issue is the setting overlay takes up half the screen which makes it hard to watch the video while listening!
seth < at > meranda< dot > org