Audio in Off Grid is largely driven by Wwise. However certain elements of the Audio are handled by Unity's integrated audio engine.
When it comes to modding Audio there are several things to try and follow.
Wwise should *Always* control Music and Atmos. There is a pool of calls (link) you can use to trigger wwise sounds in vaying different ways.
If you are creating a cutscene or similar in Timeline, the Music and Atmos should be handled by wwise still using this playable (link). Any other dialaogue or one off foley can be triggered using Unity's timeline audio track. This will help ensure you can blend between interative and cutscene sections with seamless audiom and still make use of Wwise's smart hanfling of the music etc.
Video, can be loaded into several places in the game, as described (here - link) and (here - link). As mentioned the video should have it's own sound to ensure that the 'prepare' time for the video doesn't put the audio out of sync. However you can use either Unity audio or wwise on timeline to create overlap and both sources will be able to play at the same time.