I can't tell from your photo what the color of the wires on the current receptacle are. But from the schematic, it looks like the switched side of the mains power in goes to the purple wire on the transformer.

Since for polarized receptacles, you want the hot to be the switched side of the outlet, that's the line I'd hook up the hot to. And since you're asking about the sound outlet specifically, that would be the green wire on the other side of "game over re." from the purple wire.

Does that answer your question? I might not have understood exactly what you were asking...sorry if that's the case.

Yes, that makes sense to me, and matches what I see on the schematic.

Of course, all this assumes that the main power cord for the machine is already wired correctly, i.e. with the hot going to the main power switch via the yellow wire from the wall cord. If the main wiring is already wrong, then you can wire the sound outlet up correctly but wouldn't get the actual benefit of a polarized outlet.

FWIW, it sounds like from your original post that you've already fixed the main cord. So I guess you just double-check that, unless you're already sure you did that correctly the first time. In addition, if you wanted to, you could probably add a ground to the sound outlet; just tie that back to the same ground the main cord goes to (assuming it does...the schematic shows a ground, but not a prong on the plug; since you mentioned the old one had the ground pin cut off, I'm guessing you replaced the plug with one that has a ground pin, but it's hard to know for sure based on your description).

The schematic doesn't show any ground for the sound outlet, but I don't see any reason you couldn't add it if you wanted to.