I’m running Ubuntu 21.10 and the distribution only has Audacity 2.4 so I just downloaded the latest appimage.
Audacity ‘works’ but no UI controls are visible. No sliders, no progress bars, no menu selections. I have tried all of the different themes. I manually edited the config file, as the blend with system theme option’s slider is invisible.
For me it was caused by a theme mismatch between Ubuntu and Audacity. I had Ubuntu set to dark, but I had to go to Preferences → Interface and then set the theme to match. Tested it out and the same issue appeared when I had Audacity set to dark and set Ubuntu to light.