Did you try manually executing this script as root user? This could be the difference between it succeeding and failing when run through the button.
It may be helpful to redirect the stdout and stderr of jackd and sclang to some files you could access and investigate, by adding this to the commands:
/usr/bin/jackd -P75 -dalsa -dhw:1 -p64 -n3 -s -r48000 > /home/pi/jack.log 2>&1 &
...
sclang -D /home/pi/mycode.scd > /home/pi/sc.log 2>&1 &