I'm running Mint 18.3 Mate on an Intel i5-8600K desktop. I do not have a sound card or speakers. I have a Sennheiser headset that works fine when I listen to something off the web like youtube.
I have a USB piano keyboard and sometimes it works, sometimes it doesn't. I'm using QjackCtl and Qsynth.
When I push a key on the piano, the green light in the lower left corner of Qsynth lights in unison.
In Qsynth, if I open Channels and push a key on the piano, the green light next to Yamaha Grand Piano lights. I do have two soundfont files loaded.
It has worked fine. Then it stops working. I have been searching a lot; what I've found says it should work, as far as I can tell.