Using an opamp to reduce voltage of audio signal
It took a while, but I found a copy of the datasheet for the SN76494

This diagram from the datasheet shows a buffer amplifier for the audio in on pin 9.



I find no mention of the input impedance of the IC, but with the buffer there it would be reasonable to assume a fairly high input impedance.   Assuming it is at least as good as the ancient LM741, the input impedance ought to be several hundred kiloohms.

A relatively low impedance voltage divider should do just fine.  An opamp shouldn't be needed.



Assuming your processor operates at 3.3V, and the maximum current from a GPIO pin can be 25mA, then a voltage divider with a total resistance of 132 ohms would be allowed.

You probably don't want to go that far, though.

A voltage divider of a total 1.3k ought to be low enough to work without unduly loading the GPIO.

So, a divider using a 1.2k and a 390 ohm resistor ought to do.  That gets you about 0.8V out of 3.3V, and draws about 2mA from the GPIO pin.