IC1 pin 5 is NC and shouldn't have any voltage on it and it's left floating. The build doc lists pin 8 as 'ignore'—it's a part of the offset nulling with pin 1, which aren't being used here.
Brian: Are the voltages listed in the build doc correct? Those don't seem right to me.
Update: mystery solved! 🎉 It dawned on me that after swapping the optocouplers out of desperation (with the OPA134 installed), I didn't bother to actually plug the thing in and play it because I thought the IC1 voltage readings were still wrong. The readings were closer to matching the build docs with the CA3130EZ, so that's when I gave it another spin and first noticed that the envelope effect was working.
Well... I just put an OPA134 back in and despite the voltages being "wrong" relative to the build doc, the thing works! No scratchy noises on the volume knob either. So a summary of the actual issues I encountered:
1. I missed a solder joint on initial assembly.
2. I either mis-oriented a NSL-32R3 on initial assembly (or installed a defective one... but probably the former).
3. I got kind of thrown by the IC1 voltages in the build doc, which don't seem to be correct.