Play Movie On iWatch

I have some mp4 movie files. I can get them on to my iPhone and play them in the TV app. I want to play the audio from my iWatch 4 while swimming. I don’t care at all about the video. Any way to get this done? Simplest way would be if my iWatch can just play then of my iPhone or if I can upload them directly from my phone or computer to the watch.

specific instructions appreciated. Seems like iOS is pretty finicky for non-Apple store files and I don’t have much experience with it.

Thank you.