If you are trying to burn DVD then I'd recommend to add swap space(file backed if nothing helps) and rise tmpsize="800m" to something higher.
For a DVD there is no need to increase the tmpsize since burning can be done on the fly.
I tried to burn the audio CD again with tmpsize="2000m"
and this time it worked flawlessly.
The question now, is what has to happen next. File a report at the KDE Bug system, because K3b does not give a proper error message or should the default tmpsize be changed in PC-BSD?