For your close up photo it is now quite clear that the 4 pins header is indeed for connecting external speaker. The BC55 is an NPN transistor, and the circuit is a typical common emitter switch, which sinks current to ground when it's turned on, like this:

The 2 "680" (68 ohms) resistors in parallel form Rs in the circuit, and there is no Rp (which is fine). These 2 resistors are in parallel to increase the current handling capability since each 0805 chip resistor is only 0.1W. The transistor output is connected to pin 1 of the 4 pins header, and is routed to the on board SP1 (the speaker that you broke) via the jumper to pin 2. If connecting an external magnetic/piezo speaker to pin 1 & 4 (pin 4 is Vcc) of J27 and you still have no sound, then there is probably something else that's broken. Leaving the broken pins in the holes on the motherboard do not affect anything, and the board won't "think" there is still a speaker.

To fix this, you'll probably need to find a friend who knows about electronics to help you troubleshoot. Basically you'll need to check the traces, the value of the resistors, the transistor, and the supply voltage with a multimeter. If an oscilloscope is available, it would be best to check the signal at both the base and collector of the transistor.

Think you may have your circuit wrong. From Q9 (not BC55, that is a SMT cap hidden by Q9) the out goes to parallel 680 (and C179) then to pin1 then on to speaker thru pin2 , there is no Rp (as you said) and Rs is between Q9 and the speaker. Just my observation 😀

The component hidden behind Q9 looks more like a chip capacitor than a chip transistor, as a transistor would not have such small and elongated footprint. So yeah you're right BC55 is not the transistor but a silk screen for a cap, like the one at the top left corner (BC181). Anyway even if it's Q9 the circuit is still the same, as you can see from SP1 that the +ve end is at the other side, and the pin from SP1 connected to pin 2 of the 4 pins header is -ve. This is still an NPN common emitter circuit, which is a very common PC speaker output circuitry. 😀

kalohimal wrote on 2020-08-24, 03:27:

mb3.jpg For your close up photo it is now quite clear that the 4 pins header is indeed for connecting external speaker. The BC55 […]
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mb3.jpg
For your close up photo it is now quite clear that the 4 pins header is indeed for connecting external speaker. The BC55 is an NPN transistor, and the circuit is a typical common emitter switch, which sinks current to ground when it's turned on, like this:
SC0001_img0001.gif
The 2 "680" (68 ohms) resistors in parallel form Rs in the circuit, and there is no Rp (which is fine). These 2 resistors are in parallel to increase the current handling capability since each 0805 chip resistor is only 0.1W. The transistor output is connected to pin 1 of the 4 pins header, and is routed to the on board SP1 (the speaker that you broke) via the jumper to pin 2. If connecting an external magnetic/piezo speaker to pin 1 & 4 (pin 4 is Vcc) of J27 and you still have no sound, then there is probably something else that's broken. Leaving the broken pins in the holes on the motherboard do not affect anything, and the board won't "think" there is still a speaker.

To fix this, you'll probably need to find a friend who knows about electronics to help you troubleshoot. Basically you'll need to check the traces, the value of the resistors, the transistor, and the supply voltage with a multimeter. If an oscilloscope is available, it would be best to check the signal at both the base and collector of the transistor.

Thank you so much to both for all your help!
I'm really far from being an expert at electronics like you, so i was hoping to fix this in some easy way (good thing i didn't break pin 1!). I haven't asked someone to test the motherboard yet but it has been working flawless on both DOS and Windows95. I ran a few diagnostic programs inside Windows95 and there were no errors reported...
So, basically, if i connect a speaker to pins 1 and 4 it should work right away... Since it's not working, there must be something wrong with the motherboard, right? I'm guessing since i take off the jumper on pins 1 and 2, it would work with an external one but it hasn't... Maybe i should disassemble the whole thing and take some pictures of its back too...