Step One: Open the discover store (on your task bar) and search for "VLC". Download the app with a traffic cone as a icon.

Step Two: Once its done installing, now you need either your music or movie. Legally, I can't tell you how to get this stuff, so find your own way. Make sure you install it to a easy to get to location as with everything else.

Step Three: Open VLC with either the "Launch" button in discover or by clicking the steam deck logo and left clicking VLC.

Step Four: In VLC, look at the top left and select "Media" -> "Open File" -> Your movie/song.

Step Five: That's it! Now you can listen to your music or watch movies/tv on your steam deck digitally! Do note that VLC supports MANY formats such as mp3, mp4, wav, and others. You can search for a full compatability list via your browser!