update for volumio 2.799
Terra-Berry DAC 2/3 driver is still broken. only 100% volume can be played.software volume control does not work.
but
TerraBerry DAC works now with generic I2S DAC driver. Software volume control works.
volume can be set from 1 (not hearable) to 100 (loud)
Also the issue with heavy clicks/pops when mute is set with a mp3 track is gone.
so, the new volumio version is somewhat promising for me.
Correction: it is still too loud. you have to use an volume adjustable amplifier.