First, install the fluidsynth plugin for vlc:
sudo apt-get install vlc-plugin-fluidsynth
Close any vlc windows you have open, then open vlc again.
Tools > Preferences
(or just ctrl+p)
In the preferences window choose "All" for "Show Settings" at the bottom left.
Then navigate in the list on the left to:
Input / Codecs > Audio codecs > Fluidsynth
Then click the Browse button for "Sound fonts" and choose your soundfont file.
Save then restart vlc and play a midi file.
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