There are 3 different sounds inside a Duo, first we have Hu-Card sounds that are either simple bleeps or blops and simple digitized audio, then for CD gaming we have the array of Hu-Card sounds + ADPCM, advanced (for the time) digitized audio (voice/wave synthesizer) + CD-AUDIO.
So, are you missing Hu-Card sounds even when using CD? might be a 4558 OP Amp just popped for the huey sounds, or moisture took a bite on a via powering one...