Be very careful, joining two or more videos as raw files not always works, not all video/audio formats accept a new data stream after it ends, and this method will not work with different formats.
The best way of achieving this, is transcoding the files with, for instance, "FFmpeg":http://ffmpeg.org/pipermail/ffmpeg-user/2012-February/004983.html, you can run FFmpeg through "QProcess":http://qt-project.org/doc/qt-4.8/QProcess.html.