Mac – No output in right channel audio headphones

Your Mac Mini has optical audio-out, which supports multiple channels.

Apple has cleverly put a little optical transmitter inside the headphone jack (many Macs have this, not just the Mini). To turn it into something useful, you have to use a little adapter (like this one to get a Toslink signal.

From there, it gets interesting.
You say that your speakers have to jacks. I assume you mean two analog 3.5mm (headphone) jacks.

Unless your speakers have a digital or optical input, you'll need to get a receiver that can do the digital-> analog conversion. Something like this should work (there are tons of options, this is just the first one I found).
The key is to look for something with digital/optical in, and 4+ channel preamp out.

Another option: The Mac Mini has HDMI out. You can find a receiver that has HDMI input (like the one above), and 4+ channel preamp out.

It is probably some dirt in- or another problem with your sound-output jack.

If twisting your headphone-cable or repeatedly plugging it in and out of your MacBook does not work, you might fix it by doing something like (gently!) poking into it with the backside of a matchstick or trying to clean it with some cotton buds. In that case make sure you have one that does not fall apart easily or you'll end up with a cotton-filled connector.

If that does not work, I'm not sure if you are able to fix it yourself since your MB might need to be taken apart to get into the jack and clean it…