Sync NEW audio file (mastered) with already-edited timeline

First, if you still have access to the original media and the original project, it's best to work with those and render out a new version, instead of using the exported file and replace the audio in a new project/sequence.

However, having various audio files in the timeline can make it a bit difficult to synchronize a new mixed down version of the audio. So, in order to efficiently replace the audio, dump the original sequence onto the new sequence icon in your Project panel. This will effectively create a nested version of your original sequence, but all the audio will be "merged" into a single audio file on track A1.

Inside this nested sequence, place the new audio track underneath and use the context (right-click) menu to use the option Synchronize and then either delete or mute the entire track or by selecting the original audio track and removing the checkmark for Enable. Next, export the sequence and you're done.

Hope this helps.

I'm sure there might be better ways but I would take the original full length video with the bad sound and put in into a new sequence. Sync the new audio and deleted the not wanted audio. Export as a new video and audio file using a lossless codec. In the Project Panel use the 'Replace Footage' option to change the original file for your new one. There may be ways to keep the origional video by making and renaming copies, you could try to see what works. Someone may well come along with a better answer.