Is the SB-300 cable the original and if so, has it been re-pinned? There's always a chance that could have been working fine before and disturbing it just exposed some issues with fatigued or oxidized/corroded pins. Also verify it's not off by one pin.. if it's a ribbon cable variety, I don't think those have keying pins so easy to miss a pin when reconnecting. Even the cables that do have keying pins could have the non-keyed cable shifted. Just something to check, along with perhaps having someone trigger the switches where sounds are messing up while you slowly wiggle the cable slightly up and down at the MPU J5 and SB-300 J1 connectors.. if sounds return, then it's likely the cable.