The Alesis control board normally has no sound engine. The idea is to add this sound engine. 
The wide spectrum of synth parameters is controlled directly from keyboard with MIDI/select button. Here's how it works : 

This quick hack taps the internal TTL level MIDI signal of the Alesis. This is fed into the S1 SAM2195 breakout board, which generates line level stereo audio output. See the photo's for details.   It is an improvized 5 minutes hack, here's a brief description :

"air" montage RC timing 47uF 1k ohm, low signal diode for discharge.

invertor made BC238 collector to TTL MIDI emitor to ground base thru 4k7 ohm resistor directly to MIDI output.

TTL MIDI to 5V thru 2K7 ohm resistor.

The double Led is to lower voltage only (2V drop each).  

See the schematic below :

Connect a 3.5mm jack and test. If all works fine, carefully drill a hole in your synth's case and mount the 3.5mm connector. You're done, the keyboard has turned into a synth !