No notes in piano roll ?
Volume turned right down ?
No Soundfont .sf2 file allocated ?
No instrument allocated ?
Could be anything. A few more details might help, like what sf2 track, where from ? What operating system/LMMS version ? Do other instruments like triple Oscillator make sounds normally ? Is there a soundfont set up in Edit/Settings/Paths/Default Soundfont File ? Is Audio Interface in Settings set to SDL ?
Steve