Bug in Synthesizer in MuseScore.com?

Switching to the audio source "Synthesizer" does not produce any sound on playback (Safari Browser, MacBook Air MacOS 10.14.2). Using MuseScore.com on Windows 10 via FireFox, the synthesiser does work but the playback is very jerky. The facility is essential because I am producing rehearsal tracks for a choir and they need to be able to vary the volume of each part, so synthesiser is needed. Try scores stored under twculpitt.