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It looks as though you are using a 4 channel remote control to send a "trigger" signal to each of the 4 trigger inputs of the sound board,
and play 4 different sounds using the transmitter.
https://www.adafruit.com/product/1096
Apparently, the signal out from the remote control receiver is the opposite polarity needed to trigger the sound board inputs, so it goes through the 74HC04 Hex inverter to invert the signal to the correct polarity, then to the trigger inputs. The inverter operates at 5v nominal.
https://www.technobotsonline.com/74hct04-hex-inverter.html
Before you plug the inverter into the breadboard, you can manual trigger the inputs of the sound board by hand (with a piece of wire) to see if the sound plays.
Not sure if you sound triggers on a HIGH or LOW signal.
It is possible the remote control puts out the opposite that is required. That's why the inverter is there.....
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