External sound card, stereo sound to all speakers I have an external sound card, it's SWEEX 7.1. If I plugged in, it shows at Audio MIDI setup and I can choose number of speakers, so I choose 6 speakers, because I have 5 + 1 speakers connected in my sweex sound card. I tested it by clicking to every speaker in midi setup and all speakers work correctly, which is fine, but...

What if I want to play stereo sound through all speakers? If I play some music from youtube for example, I can hear only two speakers.

So I've done some googling and found something called JackPilot, which allows me to do what I want, but you have to start JackPilot server first and after that set routing for every application separately.

I am looking for a some unified solution which will work after reboot without additional configurations. I guess Mac OS X does not provide such a feature out of the box, but I would be happy if something would exist, e.g. some 3rd party application.
Aggregate Device

You can setup an Aggregate Device (Audio Midi Setup > Create Aggregate Device), which allows the ability to combine multiple devices of the same model, or to combine multiple inputs and outputs for apps that don't support separate input and output devices. This Apple guide shows how it works and is surprisingly quite easy to setup. 

SoundFlower

Another way to route audio to multiple channels and outputs (up to 64) is with the free app/plug-in Soundflower. You can download a ready-to-use version, or compile the source code yourself if you have the desire to specifically modify the application. Soundflower is really excellent for many things — what is up to you.