From my experience with a Logitech G435 wireless headset (though no Steam Machine), you will be just fine plugging the dongle into a USB port and using it, even 30 feet away (so long as there is no interference).
I'm also aware that Steam has a built-in audio service, ao presumably you will be able to use it on SteamOS, though I imagine discord is still going to be the main way for people to communicate through VOIP (considering SteamOS is judt that: an OS, I imagine you can install discord without issue).