I may be a bit late to this party however I got here as I was wanting to know the same thing
So for the benifit of others who end up here my workaround was to split the audio from the video then copy the audio part to the number of audio tracks I had in the MP4 to new tracks.
I could then right click and select "Audio lanugages" and set each tack.
Then using the Audio mixer room set my levels.
I was using PD16.