What package to install to play midi files in Ubuntu 16.04?

Open the terminal and enter:

sudo apt install timidity timidity-interfaces-extra

Enter your password, another prompt may appear type, Y and hit enter.

Go to your menu, you should now have an application called TiMidity++ launch it. In terminal would be timidity mymusic.mid.

In TiMidity++ go to File > load and redirect it the location of your midi file, select the file, then okay then play

VLC media player can do that for you given that the plugin for midi is installed.

sudo apt install vlc vlc-plugin-fluidsynth

After that you can play your midi library with VLC without installing timidity.

The gstreamer method of playing MIDI is to use the sound card's sequencer to do so. However, many sound cards do not necessarily have an instrument for playing MIDI, these days. Instead, you will need to install a software sequencer, such as timidity to play the files.