I installed X-Plane 11.21 demo on Ubuntu 18.04. Works great except that there is no sound.
I had PulseAudio disabled, and sound worked fine for other applications. To rule out any sort of changes from packages I had added, I installed Ubuntu 18.04 fresh. Still no sound.
This is with i7 CPU and ATI FirePro V4800 video card with DisplayPort to HDMI adapter.
Steve Dunnagan