I would love to fix them for you, but shipping both ways over the pond makes it not worth it.
Check your -5v voltage. Super Cobra uses -5 in the sound output for the preamp opamp at location 3A.
The sound issue will almost certainly be on the top CPU board. Check the AY sound chip first. I've fixed a number of Stern/URL/Konami boardsets over the years.
Still looking for a repair tech that cost is less than value of board.
Not trying to be cheap, just does not make sense to repair with higher cost than just buying a working board.
Thanks
With the Scramble/Super Cobra boards the sound issues are often just the amp. Save yourself a bunch of shipping and repair fees and just shotgun the amp.
M51516L is a Mitsubishi Single Channel Audio Power Output Amplifier, 12 watt, SIP-9 Package. used on arcade game PCBs such as Konami Frogger and others.