This is a synthesizer. It outputs sounds on stdout, so to hear what it actually plays, do
./synth filename | aplay -f S16_LE -c2 -r44100
where filename is a script that contains lines like
# declare an oscillator named `name`
sinus|square|saw|triangle name
# set the note of the osc `name` to `note`
set_note name note
# merges inputs `i1`, `i2` and `i3` into `out`
merge out i1 i2 i3
# create an envelop `name` that takes `input` in
adsr name input
# sets the attack of the envelop `name` to rise to `percent` of amplitude
# at `time` milliseconds after creation or last `reset`
# The same exists with `set_decay`, `set_sustain` and `set_release`
set_attack name time percent
# resets the envelop `name`
reset name
# play the `name` for `ms` millisecs
play duration name
The lines will be read in order. If filename is unspecified or -, those
lines will be read on stdin and played on EOF (press ^D).
Look at examples.