We just wanted to let those following this thread know that we have a fix for the audio syncing issue which will be our in the next release of Camtasia Studio. While I don't know the exact date of that release, it should be out by the end of Q3 of this year.
In the meantime, if you are experiencing a sync issue with long videos on your timeline, we have found adding a split to the clip every few minutes will eliminate that problem. While this is not a fix for the bug causing this, it is a workaround that seems to work for most people.
We apologize for the issue and and trouble this has caused. Please be aware we working on resolving this issue and are committed to getting it out to you as soon as we can.