Don't really know how to make a capx out of this problem.
During the c2 contest, I added sound at the end of the process and when I tried the export, no sound played.
As there didn't seem to be bug report about that, I thought I did something wrong (audio format, tag use, etc), but didn't find anything.
Then I closed c2.. reopened it... exported... oh! it works!
I report this bug because it happened again from but it's not that often.
c2: r64 64bits
system: windows server 2008 r2
browser: firefox 7.0.1