From http://home.san.rr.com/rbarnes/Arcade/fix.txt:

****Zaxxon (+Super Zaxxon)

==Problem:  Sounds of ship flying, explosions, etc. may be there when
the game is first turned on, but slowly fade over several minutes.
Will not return until the game has been left off for a while.  [COMMON]

-Solution 1 (easy):  Replace all 2.2 uF/50V caps C69, C68, C29, C28,
C38.  Only use basic cheap-o polarized electrolytics (e.g. Mouser
140-XRL50V2.2).  Don't get fancy, as the circuit relies on the leakage
through this cap.

-Solution 2 (better):  Replace C69, C69, C29, C28, C38 with non-polar
electrolytics (e.g. Mouser 140-NPRL50V2.2).  Next, strap the 'output' side
of each cap to the +6V common though a 100kohm resistor.  Refer to the
circuit diagram to find the best place to install each resistor.

-Discussion:  These caps are used as DC blocking caps in the audio section.
As the feed a high-Z load without any tie-down resistor, they do not really
work as high-pass filters as planned.  The capacitor can build up a charge
which eventually biases the OP amp to the point of clipping.  When the caps
are fresh, leakage through the cap can prevent the run-away bias.  But if
fresh caps are installed that do not have enough leakage, they will not
work.