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I don't know what could be happening.
Try reinstalling it from the console.
Maybe the binary points to dependencies that should be pre-installed.
(There should be no need to move the plugin from .dssi. It works correctly for me as I have indicated.)

sudo apt-get -y install fluidsynth-dssi

I also provide you with this thread where different ways of working with sf2 in different plugins are discussed.

https://www.rncbc.org/drupal/node/2577