This and the sequencer itself are super impressive! I didn't even realize synthesizing sounds was possible in Decker.
Decker's audio format is basically just an array of numbers! So it's not too complicated to write a function that generates a waveform and then plays it back as audio. That's basically how my synthesizer stuff works, although obviously once you start putting things together it can get pretty complicated.
If you look at the sound interface section of the Decker documentation there's some basic sample code, that's what got me started http://beyondloom.com/decker/decker.html#soundinterface