Given that Reactor uses only two 2732 ROM chips for music, it is an amazing feat that Reactor had such awesome sound and music (in mono, no less!).  The following information was posted on RGVAC by DreamZ2048 on 6/17/97 who sent an e-mail to Dave Thiel, the wizard responsible for Reactor's killer soundtrack.  This was Dave's response:

Reactor was done with software synthesis.  The 'DSP' was a 1 MHz 6502
that ran algorithms that created a sound data stream that was slammed
into an 8 bit DAC as fast as possible.  There was no hardware timebase
so sampling frequency was a function of the complexity of the algorithm
that was making the sound data. If a loop had a branch in it you had to
take care that all the possible paths would execute the same number of
cycles (else your sampling frequency would change (and your pitch would
vary).
The trick in making something that resembled a distorted lead instrument
with such sparse resources was the waveform that I used for the lead
instrument was not in ROM.  I pointed the synthesizer algorithm at the
128 byte of RAM that was my temporary variables, global variables and
stack and used that as my waveform.  I was playing my variables as an
instrument.  Since as a matter of course they were changing (in order to
run the algorithm) the instrument sample were dynamic (and related) to
whatever note I was trying to play.
The rest of the sound effects were algorithms too.  The curious thing
about games of that period from the American pinball companies
(Williams, Gottlieb), they both used the same technique, 8-bit processor
synthesis.  Even though some sound events had as many as four
components, all sound events were mutually exclusive.  Imagine, in
Robitron for instance, there is never more than one sound event at a
time!.  And it sounds so rich and dense.  If a new sound truncates a
sound that is playing, our mind goes, oohh, this new sound is loud ( not
'wait a minute, what happened to the sound that was playing).  So much
for polyphony.

Click here for a Real Audio File of the main Reactor Theme.  "Warning!  Core Unstable!"

For more info on Reactor and other Gottlieb boards, try Lawnmower Man's Gottlieb Info Page