Just curious as to how you are doing the swap. Are you just muting the original track and adding the new track?

There is an option to detach the audio from the Video MP4 file in FCP. So I detach the audio and sync with new Audio. I the compare the wave form visually as they are stacked on each other. Sometimes its a simple swap i.e Audio in the video and audio from itunes are at the same BPM. But other times the video will contain skits or can even be at a different bpm to the CD/Single version of the audio, so i will edit the video to fit as best as i can.

You can also mute the original audio of the video in FCP

iirc, you can do that in mpeg streamclip

@skinnyguy what can I do in mpeg streamclip?

replace audio file in a video container with no degradation of video

actually, it might be ffmpeg

@skinny guy will that be "demuxing"?

i believe this will work if the Original Video and New Audio are the same length and Bpm.

Correct me if i'm wrong

i believe so. isn't that what you're doing? basically replacing the crappy audio with a better version? as you stated earlier, a straight audio swap?

this way you don't have to worry about re-encoding video and degradtion.

As long as the replacement audio has exactly the same length/speed/start time, that's a good way to do things.

I've had the least success doing that with some very old videos.

Marv, I see that no one ever answered your original question, were you able to figure it out yourself?

I am probably at a similar point you were when you started this thread. I am trying to replace the audio track on some videos. I downloaded the trial version of FCP and managed to delete the old audio track and replace it with a new one. I use uncompressed music and want to keep this when exporting. However, it seems to me that this is not possible when exporting unless using a ridiculously high video quality setting, because video and audio export quality settings can't be independently chosen. So this would mean having to convert using Handbrake or whatever, this makes no sense to me. Compressor is not an option since I most likely won't be buying FCP when the trial expires.

So here are my questions:
What does the synchronizing you mention in the first post do exactly?
Did you end up using other software to do this? Perhaps MPEG Streamclip which was mentioned?
Finally, and in general, when video and audio BPM don't match, does it make more sense to edit (stretch/compress) the audio or the video? For me the audio quality is much more important, the video can just stay the way it is, around 30 MB for some web rips. However, I'm afraid that every re-encoding of video will result in a huge increase in file size, so perhaps it's better just to edit the audio.

If there are any other threads on this topic please post them, I am either incapable of using this forum's search function or it's completely useless.

When you say uncompressed do you mean Flac and Wav files? If this is the case I'm afraid you have to convert to AAC . 320kbps is what most commercial pools use.

Synchronize is an option in FCP that allows you to sync two files eg say for example you are recording a video demo and the audio is caught from two sources eg stand alone mic and perhaps the mic on your camcorder. You will want to use the standalone mic recording as the audio for your video. You import both files into FCP. Then select both files, in this case web video and your Audio file. Then when you right click you see an option that says sync file using audio. Do this and in the same file Explorer you will see another video created which will be a combo of the Web video and the new audio but mind you the original videos audio is still included.

Open that new video file (synchronized) in FCP time line then it will show like a multitrack arrangement of the OG video + audio and the new audio track.

Then check if the peaks of both wave forms line up. (i.e Audio of the Web video and your better quality audio) If you are happy that its properly synced, then you can delete just the audio of the Web video or mute it.

Then click export to Apple Res file.

You will end up with a huge mov file, then open up Handbrake and now convert that finished Mov to MP4.

What I do in the situation when the Web video and new audio doesn't match eg, say they were recorded at different bpm you can retime the Web video and see if that makes helps.

You have to sync them first and open the synced video in FCP time line, then you might notice the two audios starting at the same point but at some point the Web videos audio starts drifting.

Sometimes I just stretch or compress just a part of the video or do some chopping. I think it's the visuals that you should work on, you don't want to be messing with the audio.

Usually I leave the audio of video attached whilst editing to serve as reference. Then before exporting I mute or delete it thus exporting with the better quality audio file I attached.

I always look for the best quality video to use in this, the sources you use probably have HD videos and best you grab that to work with.