The sketch I wrote will only make tone-like sounds. For drum-like sounds, envelope control is needed also.
Maybe add SPI control of a quad DAC with separate Vref inputs for each DAC.
http://www.analog.com/static/imported-files/data_sheets/AD5334_5335_5336_5344.pdf
The created tone feeds the Vref, the DAC will act as a digital gain control so the envelope can be manipulated.
Fast rise time, asymptotic fall, for instance.
notexpin connects to the Vref inputs, the op-amp mixer goes after the quad DAC.