I just installed PowerDirector13, (was using PowerDirector9), and had some same problem! First all worked fine, then lost sound after producing an MP4 project launching PD13 program a second or third time. It was a Sony Video Digital 8 tape Camera(Dv-Avi clip) captured using PowerDirector9
On my system, laptop Dell Studio 1749,
I had installed a Window7Codecs earlier this summer in order for Windows Media Player to play MPEG-4 MP4 files created on my LG3 Smartphone. It was the Windows7Codecs install was causing this problem - as well as bogging down the speed of PD13. I removed the PowerDirector and Windows7Codecs programs from my laptop system, then reinstalled PowerDirector13. All worked fine except my Windows Media Player could no longer play the (video) of the MP4 files from LG3 phone. So I created a system restore point and then reintroduced the Windows7Codecs - Same problem occured so i restored back to system without the Windows7Codecs. So far, none of these problems has reoccurred.