VECTOR: Battlezone problem

I still have a problem with my Battlezone that I can't seem to figure
out.
When the game is playing, everything is fine. When the game is in
attract mode, it continually plays sounds (most of the time it's the
saucer hit sound, repeating as fast as possible). When I look at the
"sound enable" line, it's always high (during game, attack mode, etc). I
thought that maybe the latch at M2 was bad, but when I trigger my scope
on the "/output latch" line and watch the input line (EDB5) to the
latch, it's always high when it gets latched. But there is activity on
the EDB5 line at other times, so I know it's not stuck.

It almost seems like this is a problem in the code, but I don't get any
kind of checksum error in test mode.

Any suggestion as to what might be the problem.