it is bus driver. the common usage - to attach/detach some signals from one part of device to another one or to/from external device. and to raise output current abilities of some low current outputs and to isolate expensive chip(like processor here) from output jack(better to ruin cheap bus driver than main processor). Ax - inputs, Yx - outputs, Cx - on/off pins. if Cx is in off state(high level on Cx) - Yx is in high impedance state and element is detached from output.
all output pins are of interest, if chip gets hot. because is some output isnt in high impedance state and is shortened somewhere - all chip will be hot. so all Y pins must be checked if they are not in high impedance.
from schematics we see that 3th element - has input (A3) connected to processor pin Fout*, C3 connected to the ground, and Y3 goes to jack. seems low C3 enables element and it simply repeats signal from Fout*, driving jack.
seems all Cx of the chip are grounded, so all elements are enabled. so any shortage on Yx, if appropriate Ax is high(seems it is not inverter), can get hot the chip. so it is good to measure voltage on Ax. if it is high(more than 1 v) but Yx is low(zero volts) - it is wrong.
imho, if his oscilloscope part is still working, better to do next - switch off generator(if it can be programmatically switched off), look via oscilloscope probe or voltmeter, if Ax is high, but Yx is low.
imho, because schematics has not 100-150 omh resistor from Y3 to output jack he could overload Y3 with speakers(as he said) and damage Y3 output.
regards,alys.