Zaxxon loses sound and there's a fix for that. Is Congo Bongo similar hardware in that way?
I did this solution 2 fix 10yrs ago and it worked on mine.
from an old rgvac post:
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, C68, 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.