I don't use a Mac, but I'm sure the same features are available in FCP that are in Vegas Video.
For a single cam shot, I just drop the video and its audio into the timeline, and then I add the alternate audio in there. As a rule, I never mess with the video's audio (snipping here and there, etc.) because that is my basis for how I need to correct the alternate audio.
Even though it's audio, within an NLE, its length is still based on framerates, so essentially, you're working with little chunks that are 1/30th of a second long.
I line up the audio at the beginning of a song, and then go to the end of it and see what the difference is. Typically, I just look for a significant sound in the audio - drum beats are usually easy to work with. If the alternate audio is 13 frames shorter than the video's audio, I stretch it 13 frames. Forgot to mention - you have to split the audio at the end of the song - but only on the alternate audio timeline - not the video's audio. I then recheck it and see how it compares. Typically, it still won't be in synch, but it will be much closer than it was before. I might do another stretch/reduction, and then if it's still off by a frame or two, I'll look for a spot in the song where I can split the audio and slide it over to overlap for a frame. Or if it needs to be stretched, I'll split it, slide it over so there's a one frame gap, and then drag to audio segment on the left over two frames so that it overlaps by one frame. This generally is undetectable to the ear if you do it anytime except on a drum beat. You can do it during a long note (guitar, organ, vocal, etc.), and of course, it's perfect if you can do it during crowd cheering.