The BC2 and A8 pins are always equal to 1 as they are connected to +5V.
The /A9 pin is non-existent on the AY-3-8912 model.
On the Amstrad CPC 6128 chassis diagram, we can see how the output of the 3 channels of the PSG are mixed with the tape sound before being sent to the speaker. [https://www.cpcwiki.eu/imgs/4/4a/CPC6128_Schematic.png Source]
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