The def-instrument-set needs to be set correctly for each instrument. There are no empty layouts
I addend a new keyword :audition into the PS function.
When :output is set to :audition no midi player or notation viewer will display - just audio.
Example:
(ps 'gm :sq (list vn1 vn2 vla vlc) :tempo 68 :output :audition)
def-score is more flexibile for it - no layout needed.