Plex is a free app that lets you stream your media collection to every device you own. Plex will convert and stream video files and music files. It creates optimized versions for mobile devices.
For downloading and streaming media I recommend this system:
Here are some reliable audio players — I’ve given priority to those that support high-fidelity playback:
This is a guide to reliable privacy-friendly services and applications.
When you are traveling — take it all offline. I review some of the best offline apps.