Hi mate,
I had the same problem. You may have found the solution but I thought I’d post for others, like me who were looking up this error.
When you hit export, make sure you have the MAKE SELF-CONTAINED MOVIE box is ticked. I have been doing heaps of exports then suddenly it stopped working and I was getting all audio. Turned out I that box had become deselected. So make sure that is checked and it should work fine again. 🙂