NE555 Eight-tone Electronic Keyboard

After a month and four training sessions, the electronic keyboard activity has concluded. In this session, 17 students' keyboards worked properly, while 3 keyboards wouldn't play.
Since everyone was using JLCPCB for the first time, various problems arose, and almost everyone became a "flying wire expert." However, there were also frustrating moments. When students' keyboards didn't work after soldering, I checked them. The schematics and PCB designs were fine, and I checked them repeatedly with a multimeter but couldn't find the problem. Testing circuits is truly a grueling task. A few cases stand out in my memory. The first student tried four flying wires, but it still didn't work. I saw that his circuit diagram looked fine, but it still wouldn't work. Finally, I used a multimeter to check each contact on his board one by one and found that one of the chip pins wasn't soldered properly. He resoldered the chip, and it worked! What a relief! Another student spent a whole day testing without success, trying various methods. Finally, just as he was about to give up after reviewing the schematic again, I discovered that his USB interface's VCC wasn't connected. I had him add a flying wire, and it was OK. The last student's circuit only had two buttons working after the wiring was completed. He resoldered all the solder points and tested everything with a multimeter, but still couldn't find the problem. However, since two buttons worked, the issue must be with the switch components. Finally, using circuit knowledge, he determined that a resistor might be faulty. He removed the resistor and connected the wires, and it worked! That's when he realized the textbook knowledge wasn't entirely useless; he just didn't know how to apply it.
This circuit has now been verified. Feel free to build your own circuit board. If it doesn't work, check the problem using the methods described above.