See if a 1k resistor is enough to cut out the onboard... if not try ~500 I think 470 is what you'll find easiest. You could keep going lower until it works, but it might be really weak signal by the time you have it across the hookup cable.

edit: by the time you drop below 50 ohms though you might be needing to check the wattage of your resistors, the tiny ones might only by 0.125W and could be getting hot by the time you hone in, half watt max should suffice at 8 Ohm and if it's not working by then it ain't gonna.

I have a SB16 MCD with the DSP (for QSound effects 🤣), bought in 1994. Still working fine, but I do not use it a lot this day. I was using it in a IBM PS/1 2133-571 (486DX33). I redirected the PC Speaker to the sound card and there was no bip on the start. It only work after setting the volume of the PC speaker in the mixer. I suspect it is set at 0 when starting.

About connecting a motherboard speaker to a sound card, there is two ways. If the speaker is connected with Dupont pins, remove the speaker and put a cable with two Dupont plug from the MB to the SC. If the speaker is embedded, if your are lucky, the are jumpers to bypass it and to connect to the SC. With my old good PS/1, it was done this way. But I would not recommend to split the SPK output between the MB speaker and the SC.

I've tried on another Motherboard, an Asus Cuv4x
This time I'm getting the pc-spk output on both Sound card and integrated pc-speaker...
So the issue is not the sound card 😀

Now I'm trying to figure out how I can leverage this "issue". Maybe some ducktape on the integrated pc-speaker

Maybe, I didn't understood. Is that what you want? To have both the internal speaker AND the sound card to produce the speaker sound?
I ask that because your internal speakers are piezoelectric, and that can sound quite odd to the ear to have the same sound coming through amplified stereo speakers and from a piezo at the same time. And that is not considering the potential lag introduced by the whole chain of audio equipment and the shift induced...
The CUV4X seems to be a mess for PC speaker. It has an embedded audio board which is connected to the same signal sent to the soldered speaker and to the PANEL connector. And in the same time, it is possible to disable the speaker-in of the embedded sound board by moving the SPK pins of the AUDIOCODEC selector. And the Matsonic MS-7112c also have a speaker soldered that cannot be disabled. Maybe you have no other motherboard, but if your goal is what I first understood: to only have the speaker sound coming through the SB16 MultiCD, I would not have chosen any of this motherboards. So, maybe you can remove the speaker on the motherboard you want to use as the last resort solution...

Yes I want to have the sound coming from the SB16 only.
And yes, seems I've 2 motherboard with integrated pc-speaker that can't be disabled. ( my Cuv4x doesn't have integrated sound card )

For the Matsonic for some reason the sound is only coming through the integrated. For the Cuv4x it's coming from both integrated and SB16
I think I'll try to put something on the integrated pc speaker to "mute" it.

Yes. Without modifying the board, and I understand you don't want to, you can try the silencer option. First, put a sticker on the hole of the speaker. And second if its not enough, as there is the battery near your speaker you can try to cut a cube of polystyrene and make a hole inside with a knife of the size of the speaker, then cut an additional little part to fit the shape of the battery holder and put the cap on. That should do the trick. Can't help better, sorry.

Good idea 😀

Thanks

OK

I spent literally hours trying to mute this damn integrated speaker on the motherboard.
Duck Tape : like nothing : Level 90%
Self adhesive Square Rubber Pads : Level 70%
I even tried to create a mold with hot glue gun on to create something like a cap : Level 70%

I finally found a solution ! I just put a small (but powerful) magnet on the speaker... : Level 5-10% can barely hear it now.

I have put a screw into the hole on top, making it also pretty quiet and can be reversed later on unless you drive it a little too deep 🤣.