Alsa

By default sound is processed through dmix plugin which handles rate and channels conversion.

This plugin communicates directly with the ALSA kernel driver. It is a raw communication without any conversions.

To override “default” pcm device and route outputdirectly into hardware:

pcm.!default {
    type hw
    card 0
    format S16_LE
}

With settings above, there is no possiblity to play mono sound (channel=1), when there is stereo audio device.

Is a plugin responsible for rate and format conversion.

pcm.!default {
    type plug
    slave {
        pcm "hw:0,0"
        format S16_LE
    }

}