Replace Audio Track with "Radio Spot" when Batching Lots of videos

Hi.

Through Media Encoder, I am able to export hundreds of videos "automatically." Nice. I would never be able to keep up with a fully manual process.

BUT - I need the ability to replace the audio track with our standard track we use on fully produced pieces.

We have a consistent video length and use the same audio track every time. I just need to replace the audio track with natural sound with our produced "Radio Spot" track. Our track is :30, the video can run long. 

The option of using a longer audio track that would automatically duck down at the end of the video track of unknown length would be awesome as well.  Obviously, I can do this manually with Premiere, but I really need Media Encoder.