KDE Connect supports controlling the music playing in the computer from an Android phone. The interface for it can be confusing when no media players are running on the computer, because the control buttons are shown but they do nothing:

This task simply consists on adding a new textview that will display "No players found" when, after loading the list of players, it is empty.

Before doing this task you will need to build KDE Connect for Android and have KDE Connect for KDE running. You can pick the beginner task "KDEConnect: Build app from sources" if you don't have this set up.