:p I already checked in4k. Good materials there.
I do
ADSR envelopes. It's simply altering the amplitude of the waveform(s). In case you don't know, the
attack is the time it takes to reach full volume,
decay is the time it takes to go down to the
sustain level, and
release is the time it takes to shut down the volume.
Echo a.k.a. delay is rather easy too. In simple mathematical terms it's like: f(n) = f(n)+f(n-delay)*amplitudeOfTheEcho.
I've tried to modulate the frequency of the sounds as I generate/play them. It sounds ok but honestly I have no idea what I'm doing, nor how to generate something that sound like a proper drum, snare, ... :p