I just checked the schematic in the patent and the Wildfire PCB that I have. The patent schematic shows the speaker connected to 2 transistors- one on A12 and the other on F. The Wildfire PCB also has one transistor connected to A12, but the other goes to pin 32, which is shown as input line K4 in the 1979_AMI_MOS_Products doc. That doc does not show an F pin (or C pin, also in the patent).

AMI_S2000_Programming_Manual_Rev2_Jun78 says instruction LAI "discharges to VSS the K lines corresponding to the bits containing zeroes". So maybe they are using K4 as an output? That would make more sense than using an internal state of the MCU, although I've heard of games that use the carry flag to toggle the speaker.