Welcome to Android Central! Are these files that are locally saved to the phone, or are they streaming media? Which apps are you using to play them? If it's only streaming, does it happen on both wi-fi and mobile data?
I've uninstalled a few apps. The only ones that are likely to be common are Google Handwriting input and Ookla Speedtest. I also cleared the all apps' cache (something I should have done from the start).