load libavcodec in Qt-Projekt-Collection of common programming errors

To compile Youre going to want to include the FFMPEG header paths, the FFMPEG library paths,

link against the avcodec.lib avformat.lib avdevice.lib avutil.lib files (or whatever your versions are called)

Then at runtime make sure the libraries avcodec.dll avformat.dll avdevice.dll avutil.dll (or whatever your versions are called) are in the same directory or the path.