When I copy/paste multiple clips into a single clip inside quicktime, QuickTime allows me to save that "edit" as a single standalone mp4 file without having to re-encode any video. This is a very fast/efficient export and works well for quick edits where you just need to trim off material.
When you look at the final output file, it has multiple audio tracks, instead of a single. At the point where the 2nd edit point plays in the video file (where the 2nd audio track plays), the audio drops out on youtube and vimeo.
Solution? Im guessing the only solution is to do an export instead of a save, which will re-encode... but im asking the question in case anyone else knows any tricks to avoid re-encoding.