In order to fix the issue, I suggest you update VirtualMIDISynth to the latest version. After that, use Google search engine to find JClive21.sf2 soundfont and download it to your computer. Once you download it, use a Soundfont decompressor and extract the .sf2 file. Next, you need to launch VirtualMIDISynth and to add the soundfont using the + sign. The soundfont chain should appear as below:
If you still have issues, I suggest you contact the developer using the form from the official web page.