I got it to work. I have closed down iClone and rebooted many times today. Not sure if I rebooted this time but I reopened iClone and loaded the project that I mentioned in the last post. No General Sound track and I deleted the character with sound. 

I inserted a new version of the mp3 that I exported from Audacity, this time being careful that I chose to "force" mono on it, (even though the file was mono anyway,) 

First, the sound track worked all the way through. Secondly, I brought the character back in by "undoing" its deletion. The general Sound still works all the way through! (!:40 video)

So there's that. We'll see how long it lasts...

Yah I am experiencing the same problems, TopOneTone. You gave a great description of an erratic and annoying problem. I read somewhere that Sound files shouldn't be more than 30 seconds, so I broke the Sound file into four parts to no avail. Also, even if I take the other character soundtrack out, and even delete the other character so that I only have one Sound track, remnants remain of the other character. I reloaded the primary Sound track, and also re-installed iClone. Nope. I could wake up tomorrow and see it work for a minute, and then...

I had exactly the same experience - the first render was really slow, 2nd render (next day) was fast. I also have a long standing problem with iclone sound - if I have multiple characters in a scene then not all of the voice tracks work, one or two can’t be heard, but they are all audible in the final render. This problem comes and goes and I have not been able to ascertain what causes it, it does not seem to be effected by project size or content, so possibly a driver update issue. It’s very annoying and unfortunately I have no cure except to keep trying and put up with the inconvenience.
cheers,
Tony

If i render the Object the sound is fine! Second Thing i realised, the first Rendertry was slow as hell!!! Than i closed the Project, opene again and try render again, suddenly if renders fast.:unsure:

All my Sound in Iclone is gone! It started with Stutter and now Audio is complete gone. Other Things like Youtube and Netflix work. IS there an Audio Option somhwere?