Hey, I’ve wrestled with this problem off and on, too – typically when I copy a sequence from one project to another project: the video and audio are there, but maybe a day or so later, the audio will disappear. The audio clips on the timeline will be be blank, but if I use “Reveal in Project”, the source file will have both audio and video.
I found, by accident, a fix that might work for you that’s easier than relinking everything: I just open “Interpret Footage” for the problem clip’s source clip, click on “Conform to:” and then click on “Use Pixel Aspect Ratio from File:…” (basically, just making a change in one of the Interpret Footage parameters, and then undoing that change), and than click “OK”.
The audio magically returns to the clip on the timeline. Why this happens, I don’t know…but it appears to return the audio to where it shoud be.