This game is fairly simple, but the audio has a decay setup.  This is done using
a transistor and RC circuit.

When RC.7 is pulled low, the audio is at full volume.  When RC.7 is high, the
voltage on the RC decays, and thus the volume does too.  It takes about 10 seconds
for it to decay to 0.  When the "Explosion" sound occurs, it decays out but
doesn't quite go silent before going back to the ticking sound.

Ticks occur at approximately a 2Hz rate.