The basic task is:
- to receive a video track and an audio track
- to insert a certain length of black at the start of the video track
- if the audio is longer, to freeze the last frame of the video track until the audio has completed
- to insert content into the audio track
- to merge the audio track into the video track

There are a couple of complications:
- sometimes we receive just a video track, in which case we have to split off the audio Stream and use that
- the content is ultrasonic, so it's important this information is not lost (I believe we can use .aac with .mp4)