Hey 
The scene you create in Unreal Engine will need to be encoded directly from Unreal only, using any methods in the documentation.
We would prefer that participants look at the command-line encoder to add audio from within the Engine when it comes to Movie Render Queue, or add audio tracks via Sequencer. However the first option isn’t the most intuitive without supporting documentation, the only documentation that really goes deep into this is third party. With this, we decided to allow the use of post-production software ONLY for adding the audio, and we will reserve the right to ask to see a screen recording of the render within UE.
Please read the Official Rules as well as the Challenge FAQ for more information.