I listen music from my computer on external speaker (via the onboard soundcard of a toshiba tecra S1), however I don't have sound anymore one the right speaker. I'm practically sure it's due to the fautly motherboard as I had already had the problem and they change it (when it use to be under warranty.. )

When I plug in my Pioneer Dj-SE (headphones), and set its switch to 'mono', I have sound on both sides (in mono of course). I want to do the same thing for my speakers.

I said to myself that if I would set the sound on my comp to mono maybe that would work. I put "mono" in the advanced option of the windows sound panel but it doesn't change anything.
I'm using foobar and I tried Channel mixer plugin and set "output channel" to 1, but doesn't work. I tried to use the "downmix channels to mono" but even if put it in the active dsp column, "configure selected" is still not available. Also I doubt it will work..

So anyone has an idea about what does the switch of my headphones and how I can recreate it? thxxxx!

If the minijack contact is broken, no software settings can possibly make voltage appear on the broken pin. You can however make sure that the left channel contains downmix of L+R instead of just L. After that its up to the speakers to send the "L" to either one or both outputs. Using a mono cable won't help either, as it will just short the right input to ground.

Your problem is that the computer mono settings means “mix the channels and put the same audio on both.” Now both channels have the same audio signal, but nothing gets through to one side because it is that channel of the soundcard that is dead -- it doesn’t matter whether it has a stereo or mono signal, nothing travels out to the speaker.

So, you do need to make the audio mono or you will only get one channels of any stereo music, even if you are feeding that one channel to both sides. This is what happens if you use the headphone mono switch while playing stereo music. Perhaps you just hadn’t notice that a lot of the music is missing.

Next you need to feed the one working channel to both speakers. You need a “mono switch” in the circuit between the soundcard and the speakers. This can be achieved with a stereo to mono adapter plug (not switchable like the headphones, but you shouldn’t care). These exist in two formats. You need one that is stereo on the side that goes into the computer and mono on the output side that connects to the speaker cable.

If this doesn’t work properly, such as you get nothing, something withing the broken soundcard is short circuiting the signal when the output channels are connected together. In that case yo will need to custom wire an adapter plug to leave the bad channel totally out of the circuit.

It's really weird, if I put any jack output into the computer, but not totally inside, then I can have stereo sound on the laptop.

Otherwise yes that's really annoying, when I listen to a cd I know at a friend's I always realise I missed the chorus, some instruments...

Eventually I'll get an external soundcard, but in between I'll get hold of such adaptater! Thanks for the help!

You are not getting stereo, you are positioning the plug so that both speaker connections touch the one channel that is working in the soundcard. What plays on both speakes is thus identical, not stereo, and is not all the audio that is in the file (unless you have internal mono playback that mixes the two channels before they get to the soundcard).