How do I configure sound in Rosegarden?

You can ignore the warnings about needing Jack running.

What I now have running is MIDI file playback via Rosegarden and QSynth.

The process as of Ubuntu 17.10 is largely the same as here, but follow these steps:

• install QSynth apt-get install fluidsynth
• install a soundfont library apt-get install fluid-soundfont-gs
• configure QSynth to Enable MIDI Input by checking the box in Setup > MIDI
• configure QSynth to use alsa Audio Driver under Setup > Audio
• configure QSynth to use the soundfont under Setup > Soundfonts; point to /usr/share/sounds/sf2/FluidR3_GM.sf2 or your soundfont location
• allow QSynth to restart when it prompts you
• install Rosegarden apt-get install rosegarden
• with QSynth open, launch Rosegarden
• Tell Rosegarden to play through QSynth Studio > Manage MIDI Devices and set the General MIDI Device to whatever QSynth broadcasts as its MIDI input option, ie 129:0 Synth input port or similar.