Are you saying the audio is present on the timeline but silent?
Is your main clip(s) still in camrec format or separate as AVI + WAV or MP3? If still in camrec, you might try to separate them into AVI + WAV and then see if that works better.
Some of the regular experts may have more to say on this. I've been around since version 2.0 but have never had this issue.
Are you trying to convert them to other Camtasia projects?
Are you attempting to open the 5.1 projects within version 8?
Or, something else, entirely?
Since I am not sure of what your process is, I will say this and maybe it will help.
As a general rule, Camtasia will only be able to read projects from one version BACK.
So, if you are trying to open version 5 projects on any version other than Version 6, you will have points of failure.
Therefore, if you need this project to be used or edited in V8.x, you need to first install V6 and open the project there then resave it. From there, you will need to install V7.x and open the V6 version of the project and save it from there.
The V7.x project could then (conceivably) be opened in V8.x.
However, that is a very long chain of conversion and I can't say if it will work. A lot has changed in the projects since V5.
You might want to just play the recording on your system speakers and record the audio from that.
Does the actual timeline display correctly for you?
If this is not helpful, give us more information to go on so we can be as helpful as possible.