Hi

Have a look at qmmp, and how they solved audio decoding using threads.
This is based on mq3 (a qt mp3 player)

It would be a matter of doing something similar, but then handling the streams (video + audio on two different buffers, putting them out on two different outputs).

Good luck with it

Also you can have a look at vlc media player, as the last front end they use is written in QT4.

If you're doing a GPL program, you can probably use most of how they solved integrating libVLC.

cheers,
leif