First error message from VLC without snd_sf2 package installed:
MIDI synthesis not set up:
A sound font file (.SF2) is required for MIDI synthesis.
Please install a sound font and configure it from the VLC preferences (Input / Codecs > Audio codecs > FluidSynth).
No suitable decoder module:
VLC does not support the audio or video format “MIDI”. Unfortunately there is no way for you to fix this.
This worked for me, I don’t usually play MIDI with VLC but tested it now. I tested it with two local .sf2 sound-fonts (Chromium and Fluid, both GM).
Did it work for you as well? I was a bit puzzled by the second post but expected it to a be an illustration of a potential error message from not having a configured sound-font set up (or installed).